That part of optics which explains the several properties of the colours of light, and of natural bodies.

Before the time of Sir Isaac Newton, we find no hypotheses concerning colours of any consequence. The opinions of the old philosophers, however, we shall briefly mention, in order to gratify the curiosity of our readers. The Pythagoreans called colour the superficies of body. Plato said that it was a flame issuing from them. According to Zeno, it is the first configuration of matter; and Aristotle said it was that which moved bodies actually transparent. Descartes asserted, that colour is a modification of light; but he imagined, that the difference of colour proceeds from the prevalence of the direct or rotatory motion of the particles of light. Father Grimaldi, Dechales, and many others, thought the differences of colour depended upon the quick or slow vibrations of a certain elastic medium filling the whole universe. Rohault imagined, that the different colours were made by the rays of light entering the eye at different angles with respect to the optic axis; and from the phenomenon of the rainbow, he pretended to calculate the precise quantity of the angle that constituted each particular colour. Lastly, Dr Hooke, the rival of Newton, imagined that colour is caused by the sensation of the oblique or uneven pulse of light; and this being capable of no more than two varieties, he concluded there could be no more than two primary colours.

In the year 1666, Sir Isaac Newton began to investigate this subject; and finding the coloured image of an illuminated sun, formed by a glass prism, to be of an oblong, and not of a circular form, as, according to the laws of refraction, it ought to be, he began to conjecture that light is not homogeneous; but that it consists of rays, some of which are much more refrangible than others. See this discovery fully explained and ascertained under the article Optics.

This method of accounting for the different colours of bodies, from their reflecting this or that kind of rays most copiously, is so easy and natural, that Sir Isaac's system quickly overcame all objections, and to this day continues to be almost universally believed. It is now acknowledged, that the light of the sun, which to us seems perfectly homogenous and white, is composed of no fewer than seven different colours, viz., red, orange, yellow, green, blue, purple, and violet or indigo. A body which appears of a red colour, hath the property of reflecting the red rays more powerfully than any of the others; and so of the orange, yellow, green, &c. A body which is of a black colour, instead of reflecting, absorbs all or the greatest part of the rays that fall upon it; and, on the contrary, a body which appears white, reflects the greatest part of the rays indiscriminately, without separating the one from the other.

The foundation of a rational theory of colours being thus laid, it next became natural to inquire, by what peculiar mechanism in the structure of each particular body it was fitted to reflect one kind of rays more than another? This Sir Isaac Newton attributes to the density of these bodies. Dr Hooke had remarked, that thin transparent substances, particularly water and soap blown into bubbles, exhibited various colours according to their thicknesses; though, when they have a considerable degree of thickness, they appear colourless; and Sir Isaac himself had observed, that as he was compressing two prisms hard together, in order to make their sides (which happened to be a little convex) to touch one another, in the place of contact they were both perfectly transparent, as if they had been but one continued piece of glass. Round the point of contact, where the glasses were a little separated from each other, rings of different colours appeared. To observe more nicely the order of the colours produced in this manner, he took two object-glasses; one of them a plano-convex one belonging to a 14 feet refracting telescope, and the other a large double convex one for a telescope of about 50 feet; and laying the former of them upon the latter, with its plain side downwards, he pressed them slowly together; by which means the colours very soon emerged, and appeared distinct to a considerable distance. Next to the pellucid central spot, made by the contact of the glasses, succeeded blue, white, yellow, and red. The blue was very little in quantity, nor could he discern any violet in it; but the yellow and red were very copious, extending about as far as the white, and four or five times as far as the blue. The next circuit immediately surrounding these, consisted of violet, blue, green, yellow, and red: all these were copious and vivid, except the green, which was very little in quantity, and seemed more faint and dilute than the other colours. Of the other four, the violet was the least in extent; and the blue less than the yellow or red. The third circle of colours was purple, blue, green, yellow, and red. In this the purple seemed more reddish than the violet in the former circuit, and the green was more conspicuous; being as brisk and copious as any of the other colours, except the yellow; but the red began to be a little faded, inclining much to purple. The fourth circle consisted of green and red; and of these the green was very copious and lively, inclining on the one side to blue, and on the other to yellow; but in this fourth circle there was neither violet, blue, nor yellow, and the red was very imperfect and dirty.

All the succeeding colours grew more and more imperfect and dilute, till after three or four revolutions they ended in perfect whiteness.

As the colours were thus found to vary according to the different distances of the glass-plates from each other, our author thought that they proceeded from density. The different thicknesses of the plate of air intercepted between the glasses; this plate of air being, by the mere circumstance of thickness or thickness, disposed to reflect or transmit this or that particular colour. From this he concluded, as already observed, that the colours of all natural bodies depended on their density, or the bigness of their component particles. He also constructed a table, wherein the thickness of a plate necessary to reflect any particular colour was expressed in parts of an inch divided into 1,000,000 parts.

Sir Isaac Newton, pursuing his discoveries concerning colours by investigating the colours of thin substances, found that the same effect were also produced by plates of a considerable thickness. There is no glass or speculum, he observes, how well polished soever, but, besides the light which it reflects or transmits regularly, scatters every way irregularly a faint light; by means of which the polished surface, when illuminated in a dark room by a beam of the sun's light, may easily be seen in all positions of the eye. It was with this scattered light that the colours in the following experiments were produced.

The sun shining into his darkened chamber through a hole in the shutter one inch wide, he let the beam of light fall perpendicularly upon a glass speculum concave on one side and convex on the other, ground to a sphere of five feet eleven inches radius, and quicksilvered over on the convex side. Then, holding a quire of white paper at the centre of the sphere to which the speculums were ground, in such a manner as that the beam of light might pass through a little hole made in the middle of the paper, to the speculum, and thence be reflected back to the same hole, he observed on the paper four or five concentric rings of colours, like rainbows surrounding the hole, very much like those which appeared in the thin plates above mentioned, but larger and fainter. These rings, as they grew larger and larger, became more dilute, so that the fifth was hardly visible; and yet sometimes, when the sun shone very clear, there appeared faint traces of a sixth and seventh.

We have already taken notice, that the thin plates made use of in the former experiments reflected some kinds of rays in particular parts, and transmitted and reflected others in the same part. Hence the coloured rings appeared variously disposed, according as they were viewed by transmitted or reflected light; that is, according as the plates were held up between the light and the eye, or not. For the better understanding of which we subjoin the following table, wherein on one side are mentioned the colours appearing on the plates by reflected light, and on the other those which were opposite to them, and which became visible when the glasses were held up between the eye and the window. We have already observed, that the centre, when the glasses were in full contact, was perfectly transparent. This spot, therefore, when viewed by reflected light, appeared appeared black, because it transmitted all the rays; and for the same reason it appeared white when viewed by transmitted light.

| Colours by Reflected Light | Colours by Transmitted Light | |---------------------------|-----------------------------| | Black | White | | Blue | Yellowish-red | | White | Black | | Yellow | Violet | | Red | Blue | | Violet | White | | Blue | Yellow | | Green | Red | | Yellow | Violet | | Red | Blue | | Purple | Green | | Blue | Yellow | | Green | Red | | Yellow | Bluish-green | | Red | Bluish-green | | Greenish-blue | Red |

The colours of the rings produced from reflection by the thick plates, followed the order of those produced by transmission through the thin ones; and by the analogy of their phenomena with those produced from the thin plates, Sir Isaac Newton concluded that they were produced in a similar manner. For he found, that if the quicksilver was rubbed off from the back of the speculum, the glass alone would produce the same rings, but much more faint than before; so that the phenomenon did not depend upon the quicksilver, except in as far as, by increasing the reflection at the back of the glass, it increased the light of the coloured rings. He also found that a speculum of metal only, produced none of those rings; which made him conclude, that they did not arise from one surface only, but depended on the two surfaces of the plate of glass of which the speculum was made, and upon the thickness of the glass between them.

From these experiments and observations, it will be easy to understand the Newtonian theory of colours. Every substance in nature seems to be transparent, provided it is made sufficiently thin. Gold, the most dense substance we know, when reduced into thin leaves, transmits a bluish-green light through it. If, therefore, we suppose any body, gold, for instance, to be divided into a vast number of plates, so thin as to be almost perfectly transparent, it is evident that all or greatest part of the rays will pass through the upper plates, and when they lose their force will be reflected from the under ones. They will then have the same number of plates to pass through which they had penetrated before; and thus, according to the number of those plates through which they are obliged to pass, the object appears of this or that colour, just as the rings of colours appeared different in the experiment of the two plates, according to their distance from one another, or the thickness of the plate of air between them.

This theory is adopted by Edward Hussey Delaval, in his Experimental Inquiry into the cause of the changes of colours in opaque and coloured bodies. Mr Delaval endeavours to confirm it by a number of experiments on the infusions of flowers of different colours; but his strongest arguments seem to be those derived from the different tinges given to glass by metallic substances. Here he observes, that each metal gives a tinge according to its specific density; the more dense metals producing the less refrangible colours, and the lighter ones those colours which are more easily refrangible. Gold, which is the densest of all metals, imparts a red colour to glass, whenever it can be divided into particles so minute, that it is capable of being mixed with the materials of which glass is made. It seems indifferent by what means it is reduced to this state, nor can it by any means be made to produce another colour. If it is mixed in large masses without being minutely divided, it imparts no colour to the glass, but remains in its metallic form. Lead, the metal whose density is next in order to that of gold, affords a glass of the colour of the hyacinth; a gem whose distinguishing characteristic is, that it is red with an admixture of yellow, the same colour which is usually called orange. Glass of lead is mentioned by several authors as a composition proper, without the addition of any other ingredient, for imitating the hyacinth. Silver, next in density to lead, can only be made to communicate a yellow colour to glass. If the metal is calcined with sulphur, it readily communicates this colour. Leaf-silver laid upon red-hot glass likewise tinges it yellow. When we meet with authors who mention a blue or greenish colour communicated by silver, the cause must have been, that the silver used in such processes was mixed with copper. Mr Delaval assures us, from his own experience, that silver purified by the tinct retains so much copper, that, when melted several times with nitre and borax, it always imparted a green colour at the first and second melting; though afterwards no such colour was obtainable from it. The only colour produced by copper is green. It is indifferent in what manner the copper is prepared in order to tinge the glass, provided it is exposed without any other ingredient to a sufficient degree of heat. If a quantity of salts are added in the preparation, they will, by attenuating the mixture, make the glass incline to blue, the colour next in order; but this happens only when the fire is moderate; for, in a greater degree of heat, the redundant salts, even those of the most fixed nature, are expelled. It is true, that copper is mentioned by some writers as an ingredient in red glass and enamel; but the red, which is the colour of the metal not dissolved or mixed with the glass, remains only while the composition is exposed to such a degree of heat as is too small to melt and incorporate it; for, if it be suffered to remain in the furnace a few minutes after the copper is added, the mass will turn out green instead of red. Iron, the metal next in density to copper, is apt to be calcined, or reduced to a ruddy crouse, similar to that rust which it contracts spontaneously in the air. In this state, it requires a considerable degree of heat to dissolve and incorporate it with glass: till that heat is applied, it retains its ruddy colour; by increasing the heat, it passes through the intermediate colours, till it arrives at its permanent one, which is blue; this being effected in the greatest degree of heat. the glass will bear, without losing all colour whatever. Iron vitrified per se is converted into a blue glass. In short, it is indubitable, that iron is the only metal which will, without any addition, impart to the glass a blue colour; for copper will not communicate that colour without the addition of a considerable quantity of salts, or some other matter that attenuates it; and the other metals cannot by any means be made to produce it at all.

These are the principal of Mr Delaval's arguments in favour of Sir Isaac Newton's theory of colours being formed by density. Dr Priestley too hath mentioned some which deserve attention. "It was a discovery of Sir Isaac Newton (says he), that the colours of bodies depend upon the thickness of the fine plates which compose their surfaces. He hath shown, that a change of the thickness of these plates occasions a change in the colour of the body; rays of a different colour being thereby disposed to be transmitted through it; and consequently rays of a different colour reflected at the same place, so as to present an image of a different colour to the eye. A variation in the density occasions a variation in the colour; but still a medium of any density will exhibit all the colours, according to the thickness of it. These observations he confirmed by experiments on plates of air, water, and glass. He likewise mentions the colours which arise on polished steel by heating it, as likewise on bell-metal, and some other metallic substances, when melted and poured on the ground, where they may cool in the open air; and he ascribes them to the scoriae or vitrified parts of the metal, which, he says, most metals, when heated or melted, do continually protrude and send out to their surfaces, covering them in the form of a thin glassy skin. This great discovery concerning the colours of bodies depending on the thicknesses of the fine plates which compose their surfaces, of whatever density these plates may be, I have been so happy as to hit upon a method of illustrating and confirming by means of electrical explosions. A number of these being received on the surface of any piece of metal, change the colour of it to a considerable distance from the spot on which they were discharged; so that the whole circular space is divided into a number of concentric rings, each of which consists of all the prismatic colours, and perhaps as vivid as they can be produced in any method whatever. Upon showing these coloured rings to Mr Canton, I was agreeably surprised to find, that he had likewise produced all the prismatic colours from all the metals, but by a different operation. He extended fine wires of all the different metals along the surfaces of pieces of glass, ivory, wood, &c.; and when the wire was exploded, he always found them tinged with all the colours. They are not disposed in so regular and beautiful a manner as in the rings I produced, but they equally demonstrate that none of the metals thus exploded discovers the least preference to one colour more than to another. In what manner these colours are formed it may not be easy to conjecture. In Mr Canton's method of producing them, the metal, or the calcined and vitrified parts of it, seem to be dispersed in all directions from the plate of explosion, in the form of spheres of a very great variety of sizes, tinged with all the variety of colours, and some of them smaller than can be distinctly seen by any magnifier. In my method of making these colours, they seem to be produced in a manner similar to the production of colours on steel and other metals by heat; i.e., the surface is affected without the parts of it being removed from their places, certain plates or laminae being formed of a thickness proper to exhibit the respective colours."

But, however well supported this doctrine of the Newtonian formation of colours by density may be, we find the theory's main author (Dr Priestley), whom we have just now quoted, has been arguing for it in his history of electricity, arguing against it in his history of vision. "There are (says he) no optical experiments with which Sir Isaac Newton seems to have taken more pains than those relating to the rings of colours which appear in thin plates; and in all his observations and investigations concerning them, he discovers the greatest sagacity both as a philosopher and mathematician; and yet in no subject to which he gave his attention, does he seem to have overlooked more important circumstances in the appearances he observed, or to have been more mistaken with regard to their causes. The former will be evident from the observations of those who succeeded him in these enquiries, particularly those of the Abbé Mazeas. This gentleman, endeavouring to give a curious experiment high polish to the flat side of an object-glass, happened to be rubbing it against another piece of flat by the Abbé and smooth glass; when he was surprized to find, that after this friction, they adhered very firmly together, till at last he could not move the one upon the other. But he was much more surprized to observe the same colours between these plane glasses that Newton observed between the convex object-glasses of a telescope and another that was plane. These colours between the plane glasses, the Abbé observes, were in proportion to their adhesion. The resemblance between them and the colours produced by Newton, induced him to give a very particular attention to them; and his observations and experiments are as follow:

"If the surfaces of the pieces of glass are transparent, and well polished, such as are used for mirrors, and the pressure be as equal as possible on every part of the two surfaces, a resistance, he says, will soon be perceived when one of them is made to slide over the other; sometimes towards the middle, and sometimes towards the edges; but wherever the resistance is felt, two or three very fine curve lines will be perceived, some of a pale red, and others of a faint green. Continuing the friction, these red and green lines increase in number at the place of contact, the colours being sometimes mixed without any order, and sometimes disposed in a regular manner. In the last case, the coloured lines are generally concentric circles, or ellipses, or rather ovals, more or less elongated as the surfaces are more or less united. These figures will not fail to appear, if the glasses are well wiped and warmed before the friction.

"When the colours are formed, the glasses adhere with considerable force, and would always continue so without any change in the colours. In the centre of all those ovals, the longer diameter of which generally exceeds ten lines, there appears a small plate of the same figure, exactly like a plate of gold interposed between the glasses; and in the centre of it there is often..." often a dark spot, which absorbs all the rays of light except the violet; for this colour appears very vivid through a prism.

"If the glasses are separated suddenly, either by sliding them horizontally over one another, or by the action of fire, as will be explained hereafter, the colours will appear immediately upon their being put together again, without the least friction.

"Beginning by the slightest touch, and increasing the pressure by insensible degrees, there first appears an oval plate of a faint red, and in the midst of it a spot of light green, which enlarges by the pressure, and becomes a green oval, with a red spot in the centre; and this, enlarging in its turn, discovers a green spot in its centre. Thus the red and the green succeed one another in turns, assuming different shades, and having other colours mixed with them, which will be distinguished presently.

"The greatest difference between these colours exhibited between plane surfaces and those formed by curve ones is, that in the former case pressure alone will not produce them, except in the case above mentioned. With whatever force he compressed them, his attempts to produce the colours were in vain without previous friction. But the reason of this plainly was, that without sliding one of the glasses over the other, they could not be brought to approach near enough for the purpose.

"Having made these observations with plates of glass whose sides were nearly parallel, he got two prisms with very small refracting angles; and rubbing them together, when they were so joined as to form a parallelopiped, the colours appeared with a surprising lustre at the places of contact, owing, he did not doubt, to the separation of the rays of light by the prism. In this case, differently coloured ovals appeared, but the plate of gold in them was much whiter, and only appeared yellow about its edges. This plate having a black spot in its centre, was bordered by a deep purple. He could not perceive any violet by his naked eye, but it might be perceived by the help of a lens with a weak light. It appeared in a very small quantity at the confines of the purple and the blue, and seemed to him to be only a mixture of these two colours. It was very visible in each of the coloured rings by inclining the glasses to the light of the moon. Next to the purple and violet appeared blue, orange, red tinged with purple, light green, and faint purple. The other rings appeared to the naked eye to consist of nothing but faint reds and greens; and they were so shaded that it was not easy to mark their terminations. That the order of these may be compared with Newton's, he gives a view of both in the following table:

| Order of the Colours in the Plane Glasses | Order of the Colours in Newton's Object Glasses | |-------------------------------------------|-----------------------------------------------| | Order I. | | | Black spot | Black | | Whitish oval | Blue | | Yellow border | White | | Deep purple | Yellow | | Order II. | | | Blue | Red | | Orange | Violet | | Purple | Blue | | Green | Green | | Yellow | Red | | Red | Green |

Order of the Colours in Newton's Object Glasses:

Purple Blue Green Yellow Red Green Red Greenish blue Red Greenish blue Red Greenish blue Pale red

"When these coloured glasses were suspended over the flame of a candle, the colours disappeared suddenly, though the glasses still continued to adhere to one another when they were parallel to the horizon. When they were suffered to cool, the colours returned by degrees to their former places, in the order of the preceding table.

"After this the Abbe took two plates much thicker than the former, in order to observe at his leisure the action of fire upon the matter which he supposed to produce the colours; and observed, that as they grew warm, the colours retired to the edges of the glasses, and there became narrower and narrower till they were reduced to imperceptible lines. Withdrawing the flame, they returned to their place. This experiment he continued till the glasses were bent by the violence of the heat. It was pleasant, he says, to observe these colours glide over the surface of the glass as they were purified by the flame.

"At the first, our author had no doubt but that these colours were owing to a thin plate of air between the glasses, to which Newton has ascribed them; but the remarkable difference in the circumstances attending those produced by the flat plates, and those produced by the object-glasses of Newton, convinced him that the air was not the cause of this appearance. The colours of the flat plates vanished at the approach of flame, but those of the object-glasses did not. He even heated the latter till that which was next the flame was cracked by the heat, before he could observe the least dilatation of the coloured rings. This difference was not owing to the plane glasses being less compressed than the convex ones; for though the former were compressed ever so much by a pair of forceps, it did not in the least hinder the effect of the flame.

"Afterwards he put both the plane glasses and the convex ones into the receiver of an air-pump, suspending the former by a thread, and keeping the latter compressed by two strings; but he observed no change in the colours of either of them in the most perfect vacuum he could make.

"Notwithstanding these experiments seemed to be conclusive against the hypothesis of these colours being formed by a plate of air, the Abbe frankly acknowledges, that the air may adhere so obstinately to the surface of the glasses as not to be separated from them by the force of the pump; which, indeed, is agreeable to other appearances; but the following experiments of our author make it still more improbable that the air should be the cause of these colours." "To try the utmost effect of heat upon these coloured plates, after warming them gradually, he laid them upon burning coals; but though they were nearly red, yet when he rubbed them together by means of an iron rod, he observed the same coloured circles and ovals as before. When he ceased to press upon them, the colours seemed to vanish; but when he repeated the friction, they returned, and continued till the pieces of glass began to be red-hot, and their surfaces to be united by fusion.

"When the outward surface of one of his plates of glass was quicksilvered, none of those colours were visible, though the glasses continued to adhere with the same force. This he ascribed to the stronger impression made on the eye by the greater quantity of light reflected from the quicksilver.

"Judging from the resemblance between his experiments and those of Sir Isaac Newton, that the colours were owing to the thickness of some matter, whatever that was, interposed between the glasses, the Abbé, in order to verify his hypothesis, tried the experiment on thicker substances. He put between his glasses a little ball of suet, about a fourth of a line in diameter, and pressed it between the two surfaces, warming them at the same time, in order to disperse the suet; but, though he rubbed them together as before, and used other soft substances besides suet, his endeavours to produce the colours had no effect. But, rubbing them with more violence in a circular manner, he was surprized, on looking at a candle through them, to see it surrounded with two or three concentric rings, very broad, and with very lively delicate colours; namely, a red inclining to a yellow, and a green inclining to that of an emerald. At that time he observed only these two colours; but continuing the friction, the rings assumed the colours of blue, yellow, and violet, especially when he looked through the glasses on bodies directly opposed to the sun. If, after having rubbed the glasses, the thickness was considerably diminished, the colours grew weaker by transmitted light, but they seemed to be much stronger by reflection, and to gain on one side what they lost on the other.

"Our author was confirmed in his opinion, that there must be some error in Newton's hypothesis, by considering, that, according to his measures, the colours of the plates varied with the difference of a millionth part of an inch; whereas he was satisfied that there must have been much greater differences in the distance between his glasses, when the colours remained unchanged.

"If the colour depended upon the thickness only, he thought that the matter interposed between the glasses ought to have given the same colour when it was reduced to a thin plate by simple fusion as well as by friction, and that, in rubbing two plates together, warming them at different times, and compressing them with a considerable force, other colours would have appeared besides those above mentioned.

"These circumstances made him suspect, that the different thicknesses of the substance interposed between the glasses served only to make them more or less transparent, which was an essential condition in the experiment; and he imagined that the friction diffused over the surface of the thin substance a kind of matter on which the colours are formed by reflected light: for when he held the plates (which gave the colours when the suet was between them) over the flame of a small candle, the colours fled with great precipitation, and returned to their place without his being able to perceive the least alteration in the suet.

"He was confirmed in his conjectures, by frequently observing, that when the glasses were separated, at the moment the colours disappeared, they were covered with the same greasy matter, and that it seemed to be in the very same state as when they were separated without warming. Besides, having often repeated the same experiment with different kinds of matter, he found that the degree of heat that dispersed the colours was not always sufficient to melt it; which difference was more sensible in proportion as the matter interposed was made thinner.

"Instead of the suet, he sometimes made use of Spanish wax, resin, common wax, and the sediment of urine. He began with Spanish wax, on account of its remarkable transparency in Mr Hawksbee's electrical experiments; but he had much difficulty in making it sufficiently thin by friction, being often obliged to warm his glasses, to seize the moment of fusion, which continued but a short time, and to hazard the burning of his fingers.

"The experiment at length succeeding, the Spanish wax appeared with its opacity and natural colour when it reflected the light, but they both disappeared in the transmitted light. He observed the same rings in it as in the suet; and indeed he could perceive but little difference between the colours of suet, Spanish wax, common wax, or resin; except that this last substance did not make the colours so vivid, on account of the too great transparency of its particles.

"The sediment of urine had something more particular in its appearance, as its colours were more lively. Holding it above the flame, its colours disappeared; and, keeping it in that situation, there were formed, upon its surface, ramifications, like those of the hoar-frost, which disappeared as the glasses grew cold. There were the same ramifications both upon the suet and the wax, but they were not so considerable. The glasses which had Spanish wax and resin between them adhered with so much force, that they could not be separated without the help of fire; and when they began to grow warm, they separated with a noise like that of a glass breaking in the fire, though the glasses were not broken, and the matter between them was not melted.

"Separating the glasses which he first used very suddenly, he observed upon their surface very thin vapours, which formed different colours, but presently vanished altogether.

"To try the effect of vapour, he breathed upon one of his plates of glass, and observed that the vapours which adhered to the glasses sometimes formed, before they were entirely dispersed, a surprising variety of colours. This experiment, he observes, does not always succeed at the first trial. The glass must be breathed upon several times, and care must be taken to wipe it every time with one's hand, both to take off the moisture, and also to make upon the glass a kind of furrows, which contribute very much to the variety of colours, by making inequalities in the thicknesses of the vapours. It is necessary, also, that the glasses on which these experiments are made have no quicksilver upon them.

"When the particles of water which formed this vapour were too thick to exhibit these colours, he struck them several times with his pencil, in order to attenuate them; and then he saw an infinity of small coloured threads which succeeded one another with great rapidity.

"Putting a drop of water between two pieces of common glass, he observed that the compression of them produced no colour; but if, while they were compressed, the water was made to pass from one place to another, it left behind it large spots, red, yellow, green, purple, &c., and the spots assumed different colours with surprising rapidity, and presented to the eye a most beautiful variety of shades.

"In order to determine with greater certainty whether they were vapours that caused the colours in his first observations, he first breathed upon one of his plates of glass, and then rubbed them against one another, when the colours appeared in the same order as before, but darker, and dispersed in confusion in the places occupied by the vapours; but when he made use of fire to dilute the watery particles, the colours resumed their lustre.

"Newton, having introduced a drop of water between his two object-glasses, observed, that in proportion as the water infatuated itself between the glasses, the colours grew fainter, and the rings were contracted; and ascribing these colours to the thickness of the plate of water, as he ascribed the former to that of the plate of air, he measured the diameters of the coloured rings made by the plate of water, and concluded that the intervals between the glasses at the similar rings of these two mediums were nearly as three to four; and thence he inferred, that, in all cases, these intervals would be as the sines of the refractions of these mediums.

"The Abbé Mazzea, in order to assure himself whether, agreeable to this rule, the coloured rings of his glasses depended upon the thickness of the water only, dipped one of the edges of his coloured glasses in a vessel of water, having taken care to wipe and warm them well before he produced his colours by friction. The water was a considerable time in rising as high as the glasses; and in proportion as it ascended, he perceived a very thin plate of water, which seemed to pass over the matter which he thought produced the colours, without mixing with it; for beyond this plate of water, he still perceived the colours in the same place and order, but deeper and darker; and holding the glasses above the flame of a candle, he saw the colours go and come several times as he moved them nearer to or farther from the flame. He then moistened both the glasses more than before; and rubbing them as usual, he always saw the same appearance; and seizing the moment when the colours had disappeared to separate the glasses, he always found that they were wet. On this account, he thought that it could not be the water on which the colour depended, but some substance much more sensible to heat. He also thought that these coloured rings could not be owing to the compression of the glasses; or that, if this circumstance did contribute any thing to them, it served rather to modify than to generate them.

"M. du Tour gave particular attention to the preceding observations of the Abbé Mazzea. He repeated Tour's observations with some variation of circumstances, particularly comparing them with those of Sir Isaac Newton. He is so far from supposing a plate of air to be necessary to the formation of these coloured rings, that he thinks the reason of their not appearing between the flat plates of glass is the adhering of the air to their surfaces; and that mere pressure is not sufficient to expel it; except, as the Abbé Mazzea observed, the rings had before been made in the same place; in which case, simple apposition without friction is sufficient; the air, probably, not having had time to apply itself so closely to the surface of the glasses. The contact of some other substances, M. du Tour observes, is not so prejudicial in this experiment as that of air; for he found, that, if he only gave the plates a slight coating of any kind of grease, the rings would appear without friction. Also dipping them slightly in water, or wiping them with his finger, would answer the same purpose. He verified his conjectures by means of the air-pump: for, dipping two pieces of glass in water, one of which had been wiped, and the other not, the former appeared to have no bubbles adhering to it when the air was exhausted, whereas the other had.

"When one of the glasses is convex, our author observes, that the particles of air may more easily make their escape by pressure only; whereas their retreat is in a manner cut off when they are compressed between two flat surfaces. The air-pump, he found, was not able to detach these particles of air from the surfaces to which they adhere; leaving these flat plates for a considerable time in an exhausted receiver, was not sufficient to prepare them so well for the experiment, as wiping them.

"Besides the observations on the colours of thin plates, it has been seen that Sir Isaac Newton imagined on gine he could account for the colours exhibited by colours by thick ones in some cases in a similar manner; particularly in those curious experiments in which he admitted a beam of light through a hole in a piece of pasteboard, and observed the rings of colours reflected back upon it by a concave glass mirror of equal thickness in all places. These experiments were resumed, and happily pursued, by the Duke de Chaulnes, who ascribed these colours to the inflection of light*. Chance led the duke to observe, that when the nearer surface of the glass mirror was clouded by breathing upon it, so as lightly to tarnish it, a white diffused and vivid light was seen upon the pasteboard, and all the colours of the rings became much stronger, and more distinct. This appearance he made constant by moistening the surface of the mirror with a little milk and water, and suffering it to dry upon it.

"In all his experiments upon this subject, he found, that when the rays fell converging on the surface of the mirror, the rings were hardly visible; when they fell parallel upon it, as they must have done in all the experiments of Newton, they appeared sufficiently distinct; but when, by means of a convex lens placed in the hole of the window, they were made to diverge from..." from the centre of the sphere to which the mirror was ground, so that they fell perpendicularly on the surface of the mirror, the colours were as vivid as he could make them. In this case, he could remove the reflected image to a great distance from the hole, without making the rings disappear; and he could plainly perceive them to arise from their central spots, which changed their colours several times.

"The effect of tarnishing the mirror convinced him, that these coloured rings depended on the first surface of the mirror; and that the second surface, or that which reflected them after they had passed the first, only served to collect them and throw them upon the pasteboard in a quantity sufficient to make them visible, and he was confirmed in his supposition by the following experiments.

"He took a plano-convex object-glass, of six feet focus, and placed it five feet from the pasteboard with its convex side towards it. By this means the rays which fell upon that surface, after being refracted there, were transmitted through the thickness of the glass, parallel to one another, and fell perpendicularly on the plane surface that reflected them, and, in their return, would be collected upon the pasteboard. In these circumstances the rings appeared very distinct after he had tarnished the convex surface, which in this position was next to the light.

"Turning the same glass the contrary way, so that the plane surface was towards the pasteboard, he could perceive none of the rings at the distance of five feet; but they were visible at the distance of three feet: because at that distance the second surface reflected the rays by its concavity directly towards the pasteboard.

"These two experiments demonstrate the use of the second surface of the mirror, and show the manner of placing it to most advantage. Those that follow show the use of the first surface with respect to these rings; and he was led to make them by the casual observation above mentioned.

"Newton, he observes, had remarked, that when he made use of a mirror of the same focus with the first he had used, but of twice the thickness, he found the diameter of the rings much smaller than before. This observation the duke thought favourable to his own conclusions; for if these rings depend upon the first surface, the nearer it is to the second, which only reflects the ray transmitted from it, the larger they ought to appear upon the pasteboard.

"To ascertain this fact, he thought of making use of two moveable surfaces; and to make use of a micrometer to measure the distance between them with exactness. For this purpose he took a metallic mirror belonging to a reflecting telescope, being part of a sphere of ten feet radius; and he fixed it firm upon a foot in which was a groove that carried a light frame, to which was fastened a thin piece of talc tarnished with milk and water. The frame that supported the piece of talc could either be brought into contact with the mirror, or be removed to the distance of eight or nine inches from it, and the micrometer showed to the utmost exactness the least motion of the frame.

"Having placed this mirror ten feet from the pasteboard, that is, at the distance of the radius of its own sphere, he observed the rings to appear very distinct; the form of his mirror being very true: but the diameter of the rings upon the pasteboard varied with the distance of the talc from the mirror; so that they were very large when the talc was near the mirror, and very small when it was placed at the distance of seven or eight inches.

"These experiments proved, that the rings were formed by the first surface, and reflected by the second; but it still remained to be determined in what manner they were formed. He imagined, that the small pencils of rays that were transmitted through the pores of the glass, or any other transparent substance, might suffer a kind of inflection, which might change the cylinder which they formed into a truncated cone, either by means of their different degrees of inflexibility, or by the different distances at which they pass by the edges of the small hole through which they are transmitted. Pursuing this idea, he thought of making use of some body, the pores of which were of a known and determined shape. Instead, therefore, of the piece of talc, he placed a piece of fine linen in the above mentioned frame, stretching it as even as possible, to make the pores formed by the threads more exact and more permeable by the light; and he soon found, with great pleasure, that his conjecture was verified: for, instead of the circular rings which he had before, they were now manifestly square, though their angles were a little rounded; and they were coloured as the others, though the light was not very vivid, on account of the quantity that was stopped by the muslin.

"When, instead of the muslin, he stretched across his frame fine silver wires exactly parallel, at the distance of about three quarters of an inch, or a whole line from one another, without any other wires across them; instead of the rings which he had seen before, there was nothing upon the pasteboard but a gleam of white light divided by many small streaks, coloured in a very vivid manner, and in the same manner as the rings."

Thus we have another hypothesis of the formation of colours, namely, by the inflection of light in its theory of passage out from between the solid and impenetrable colours, particles of which bodies are composed. It is, however, very difficult, upon the hypothesis either of Sir Isaac Newton, or that of the Duke de Chaulnes, to give a reason why bodies that are not entirely white, should not appear variously coloured. For, it appears from Sir Isaac Newton's experiments, that plates of different density are capable of exhibiting the same colours; and that where a plate is continually varying in density, it will produce all the colours. Now it is evident, that the plates of which we suppose all natural bodies to be composed, must be similar to one that is perpetually varying in its thickness; for supposing the plates of which any substance is composed to be of any determinate thickness, 9 millionth parts of an inch for instance; such of the rays as are reflected from this plate will be red. But if any of them penetrate to the depth of $11\frac{1}{2}$ of these parts, they will be reflected of a violet colour, &c., and thus must alloy and obscure the red; and so of others. If we suppose the colours to be produced by inflection, it will be equally difficult to account for some particular rays being inflected and others not; seeing we observe serve that all of them are capable of being inflected by every substance whatever, when they pass very near it. In some cases too, colours are produced when the light is neither refracted nor inflected, as far as we can judge; and this seems to obscure the theory of chromatics more than any thing we have yet mentioned.

As the experiments we are now about to mention are of the greatest importance, and in direct terms contradict one of Sir Isaac Newton's, we shall give a full account of them, from Priestley's history of Vision, &c., with his remarks thereon.

The experiment in question is the eighth of Newton's second book of Optics: "He (Sir Isaac Newton) found, he says, that when light goes out of air through several contiguous refracting mediums, as through water and glass, and thence goes out again into air, whether the refracting surfaces be parallel or inclined to one another, that light, as often as, by contrary refractions, it is so corrected, that it emerges in lines parallel to those in which it was incident, continues ever after to be white: but if the emergent rays be inclined to the incident, the whiteness of the emerging light will, by degrees, in passing on from the place of emergence, become tinged at its edges with colours. This he tried by refracting light with prisms of glass, placed within a prismatic vessel of water.

"By theorems deduced from this experiment, he infers, that the refraction of the rays of every sort, made out of any medium into air, are known by having the refraction of the rays of any one sort; and also, that the refraction out of one medium into another is found as often as we have the refractions out of them both into any third medium.

"On the contrary, a Swedish philosopher (M. Klingenaflerna) observes*, that, in this experiment, the rays of light, after passing through the water and the glass, though they come out parallel to the incident rays, will be coloured; but that the smaller the glass prism is, the nearer will the result of it approach to Newton's description.

"This paper of M. Klingenaflerna, being communicated to Mr Dollond by M. Mallet, made him entertain doubts concerning Newton's report of the result of his experiment; and determined him to have recourse to experiments of his own.

"He therefore cemented together two plates of parallel glass, at their edges, so as to form a prismatic vessel when flopped at the ends or bases; and the edge being turned downwards, he placed in it a glass prism with one of its edges upwards, and filled up the vacancy with clear water; so that the refraction of the prism was contrived to be contrary to that of the water, in order that a ray of light, transmitted through both these refracting mediums, might be affected by the difference only between the two refractions. As he found the water to refract more or less than the glass prism, he diminished or increased the angle between the glass plates, till he found the two contrary refractions to be equal, which he discovered by viewing an object through this double prism. For when it appeared neither raised nor depressed, he was satisfied that the refractions were equal, and that the emergent rays were parallel to the incident.

Vol. IV. Part II.

"Now, according to the prevailing opinion, he observes, that the object should have appeared through this double prism in its natural colour; for if the difference of refrangibility had been in all respects equal, in the two equal refractions, they would have rectified each other. But this experiment fully proved the fallacy of the received opinion, by showing the divergency of the light by the glass prism to be almost double of that by the water; for the image of the object, though not at all refracted, was yet as much inflected with prismatic colours, as though it had been seen through a glass wedge only whose angle was near 30 degrees.

"This experiment is the very same with that of Sir Isaac Newton above mentioned, notwithstanding the result was so remarkably different: but Mr Dollond affirms us, that he used all possible precaution and care in his process; and he kept his apparatus by him, that he might evince the truth of what he wrote, whenever he should be properly required to do it.

"He plainly saw, however, that if the refracting angle of the water-vessel could have admitted of a sufficient increase, the divergency of the coloured rays would have been greatly diminished, or entirely rectified; and that there would have been a very great refraction without colour, as he had already produced a great discolouring without refraction: but the inconvenience of so large an angle as that of the prismatic vessel must have been, to bring the light to an equal divergency with that of the glass prism, whose angle was about 60°, made it necessary to try some experiments of the same kind with smaller angles.

Accordingly he got a wedge of plate-glass, the angle of which was only nine degrees; and, using it in the same circumstances, he increased the angle of the water-wedge, in which it was placed, till the divergency of the light by the water was equal to that by the glass; that is, till the image of the object, though considerably refracted by the excess of the refraction of the water, appeared nevertheless quite free from any colours proceeding from the different refrangibility of the light.

Notwithstanding it evidently appeared, I may say to almost all philosophers, that Mr Dollond had made a real discovery of something not comprehended in the optical principles of Sir Isaac Newton, it did not appear to be sensible a man, and so good a mathematician, as Mr Murdoch is universally acknowledged to be. Upon this occasion he interposed in the defence, as he imagined, of Sir Isaac Newton; maintaining, that Mr Dollond's positions, which he says, he knows not by what mischance have been deemed paradoxes in Sir Isaac's theory of light, are really the necessary consequences of it. He also endeavours to show, that Sir Isaac might not be mistaken in his account of the experiment above mentioned. But admitting all that he advances in this part of his defence, Newton must have made use of a prism with a much smaller refracting angle than, from his own account of his experiments, we have any reason to believe he ever did make use of.

The fact probably was, that Sir Isaac deceived himself in this case, by attending to what he imagined to be the clear consequences of his other experiments; and though the light he saw was certainly tinged with colours, colours, and he must have seen it to be so, yet he might imagine that this circumstance arose from some imperfection in his prisms, or in the disposition of them, which he did not think it worth his while to examine. It is also observable, that Sir Isaac is not so particular in his description of his prisms, and other parts of his apparatus, in his account of this experiment, as he generally is in other cases, and therefore probably wrote his account of it from his memory only.

Much has been said on this experiment; and it is thought very extraordinary, that a man of Sir Isaac's accurate attention should overlook a circumstance, the effect of which now appears to be so considerable. But it has happily occurred to Mr Michell, that, as Sir Isaac Newton observes he used to put saccharum saturni into his water to increase its refractive power, the lead, even in this form, might increase the diffusive refraction, as it does in the composition of glass; and if so, that this would account for Newton's not finding the diffusive power of water less than that of his glass prisms, which he otherwise ought to have done, if he had tried the experiment as he said he did.

Accordingly he included a prism of glass in water, as highly impregnated with saccharum saturni as it would bear, the proportion of saccharum to water being about as 5 to 11. When the image, seen through the water (so impregnated) and a glass prism, was in its natural place, it still was coloured, though very little; he thought not more than a fourth part as much as when seen through plain water, and the prism in its natural place; so that he had no doubt, but that, if his prism had had a little less of the diffusing power, its errors would have been perfectly corrected.

Besides the experiments of Mr Delaval above related, and which were made on the colours of transparent bodies, he has lately published an account of some made upon the permanent colours of opaque substances; the discovery of which must be of the utmost consequence in the arts of colour-making and dyeing. These arts, he observes, were in very remote ages carried to the utmost height of perfection in the countries of Phoenicia, Egypt, Palestine, India, &c., and that the inhabitants of these countries also excelled in the art of imitating gems, and tingling glass and enamel of various colours. The colours used in very ancient paintings were as various as those now in use, and greatly superior both, in beauty and durability. The paints used by Apelles were so bright, that he was obliged, to glaze his pictures with a dark-coloured varnish, lest the eye should be offended by their excessive brightness; and even these were inferior to what had been used among the ancient Egyptians. Pliny complains that the art of painting was greatly decayed in his time; and the moderns were not furnished with any means of retrieving the art until they began to avail themselves of experimental observations.

The changes of colour in permanently coloured bodies, our author observes, are produced by the same laws which take place in transparent colours; substances; and the experiments by which they can be investigated consist chiefly of various methods of uniting the colouring particles into larger, or dividing them into smaller masses. Sir Isaac Newton made his experiments chiefly on transparent substances; and in the few places where he treats of others, acknowledges his deficiency of experiments. He makes the following remark, however, on those bodies which reflect one kind of light and transmit another, viz. that "if these glasses or liquids were too thick and muddy that no light could get through them, he questioned whether they would not, like other opaque bodies, appear of one and the same colour in all positions of the eye; though he could not yet affirm it from experience."

It was the opinion of this great philosopher, that all coloured matter reflects the rays of light, some reflecting the more refrangible and others the less refrangible rays more copiously; and that this is not only a true reason of these colours, but likewise the only reason. He was likewise of opinion that opaque bodies reflect the light from their anterior surface by some power of the body evenly diffused over and external to it. With regard to transparent coloured liquors, he expresses himself in the following manner: "A transparent body which looks of any colour by transmitted light, may also look of the same colour by reflected light; the light of that colour being reflected by the farther surface of that body, or by the air beyond it; and then the reflected colour will be diminished, and perhaps cease, by making the body very thick, and pitching it on the back-side to diminish the reflection of its farther surface, so that the light reflected from the tinging particles may predominate. In such cases the colour of the reflected light will be apt to vary from that of the light transmitted."

To investigate the truth of these opinions Mr Delaval entered upon a course of experiments with transparent coloured liquors and glasses, as well as with opaque and semitransparent bodies. From these he discovered several remarkable properties of the colouring matter; particularly, that in transparent coloured substances it does not reflect any light; and when, by intercepting the light which was transmitted, it is hindered from passing through such substances, they do not vary from their former colour to any other, but become entirely black (a).

This incapacity of the colouring particles of transparent bodies to reflect light, being deduced from very numerous experiments, may therefore be held as a general law. It will appear the more extensive, if we consider that, for the most part, the tinging particles of liquors or other transparent substances are extracted from opaque bodies; that the opaque bodies owe their colours to those particles, in like manner as the transparent substances do; and that by the loss of them they are deprived of their colours.

For making his experiments, Mr Delaval used small vials.

(a) Here our author observes, that he makes use of the word colour only to express those called primary; such a mixture of them as does not compose whitenefs, or any of the gradations between white and black, such as are called by Sir Isaac Newton, grey, dun, or russet brown. vials of flint glass, whose form was a parallelopiped, and their height, exclusive of the neck, about two inches, the base about an inch square, and the neck two inches in length. The bottom and three sides of each of these vials was covered with a black varnish; the cylindrical neck, and the anterior side, except at its edges, being left uncovered. He was careful to avoid any crevices in the varnish, that no light might be admitted except through the neck or anterior side of the vials.

In these experiments it is of importance to have the vials perfectly clean; and as many of the liquors are apt to deposit a sediment, they ought to be put into the vials only at the time the experiments are to be made. The uncovered side of the vials should not be placed opposite to the window through which the light is admitted; because in that situation the light would be reflected from the farther side of the vial; and our author observes that smooth black substances reflect light very powerfully. But as it is a principal object in the experiment that no light be transmitted through the liquor, this is best accomplished by placing the uncovered side of the vial in such a situation that it may form a right angle with the window.

With these precautions our author viewed a great number of solutions, both of coloured metallic salts and of the tinged matter of vegetables; universally observing, that the colour by reflection was black, whatever it might be when viewed by transmitted light. If these liquors, however, are spread thin upon any white ground, they appear of the same colour as when viewed by transmitted light; but on a black ground they afford no colour, unless the black body be polished; in which case the reflection of the light through it produces the same effect as transmission.

The experiments with tinged glasses were in many respects analogous to those with transparent coloured liquors. For these he made several parcels of colourless glass, principally using one composed of equal parts of borax and white sand. The glass was reduced to powder, and afterwards ground, together with the ingredients by which the colours were imparted. "This method (says he) of incorporating the tinged particles is greatly preferable to mixing them with the raw materials; and the glasses thus composed excel most others in hardness, being scarcely inferior in lustre to real gems."

The result of all the experiments made in this manner was, that when matter is of such thinness, and the tinge so dilute, that light can be transmitted through it, the glasses then appear vividly coloured; but when they are in larger masses, and the tinged matter is more densely diffused through them, they appear black; for these, as well as the transparent coloured liquors, show their colour only by transmission. The following experiments were made with a view to determine the proportion of tinged matter which produces colour or blackness.

1. Glass was tinged green by adding to it 1/5th of its weight of copper; and that whether the latter was used in its metallic or calcined state.

2. A blue glass was made by the addition of saffron, a purple one by manganese, a red glass by gold, and yellow glasses by silver and calcined iron. A yellow glass resembling a topaz was likewise made by the addition of a small quantity of charcoal in powder. The same colour was likewise procured by the addition of wheat-flour, rosin, and several other inflammable matters. Small pieces of each of these glasses being ground by a lapidary, resembled gems of their different colours.

3. Having formed pieces of such glasses about two inches thick, he inclosed them in black cloth on all sides except their farther and anterior surfaces. In this situation each of them showed a vivid colour when light was transmitted through them; but when the posterior surface was likewise covered with the cloth to prevent this transmission, no other colour than black was exhibited by any of them.

4. When plates of transparent coloured glass, somewhat thicker than common window-glass, were made use of, they always exhibited their colours by transmitted light.

5. On intercepting the light transmitted through these coloured plates, they as constantly appeared black when placed in such a direction as to form a right angle with the window.

From these phenomena Mr Delaval deduced the following observations: 1. That the colouring particles do not reflect any light. 2. That a medium, such as Sir Isaac Newton has described, is diffused over both the anterior and farther surfaces of the plates, whereby objects are equally and regularly reflected as by a mirror. Hence, when it is said that light is reflected by the surface of any substance, it should be understood from this expression, that the reflection is effected by the medium diffused over its surface.

6. When a lighted candle is placed near one of those on the recoloured plates, the flame is reflected by the medium of which is diffused over the anterior surface. The image thus reflected entirely resembles the flame in size and colour; being scarcely diminished, and not in the least tinged by the coloured glass.

7. If the plate be not so intensely coloured, or so nearly, as to hinder the transmission of the light of the candle, there appears a secondary image of the flame, which is reflected by the medium contiguous to the farther surface of the glass; and as the light thus reflected passes through the coloured glass, it is tinged very vividly.

8. When the glass used in this experiment is of a green colour, the image of the flame is always of a bright green; and when glasses of other colours are used, that of the secondary flame is always the same with that of the glass.

9. The secondary image is less than that reflected from the anterior surface. This diminution is occasioned by the loss of that part of the light which is absorbed in passing through the coloured glass. For whenever any medium transmits one sort of rays more copiously than the rest, it stops a great part of the differently coloured rays. Much more light also is lost in passing through coloured than transparent substances. In making these observations, it is proper to choose coloured plates of glass which are not in every part of an equal thickness, that the secondary image may not coincide with that reflected from the anterior surface, and be intercepted by it.

10. When the plates are so thick, and so copiously coloured, that the light cannot penetrate to their farther ther surface, they appear intensely black in whatever direction they are viewed, and afford no secondary image, but only reflect, from their anterior surface, the flame, or any other objects that are opposed to them. These objects are represented in their own proper colours, and are as free from tinge as those reflected from quicksilvered glass, or specula made of white metals.

Hence again it is manifest, that the colouring particles do not possess any share of reflective power; for if they had any share in this reflection, they would certainly impart some share of colour to the light they reflected. Hence also it appears, that transparent coloured bodies, in a solid state, possess no more reflective power than those in a fluid state.

Our author next considers the colouring particles themselves, pure, and unmixed with other media. In order to procure masses made up of such particles, several transparent coloured liquors were reduced to a solid consistence by evaporation. By employing a gentle heat, the colouring matter may thus remain unimpaired; and is capable of having its particles again separated by water or other liquids, and tinged them as before.

In this state the colouring particles reflect no light, and therefore appear uniformly black, whatever substances they have been extracted from. In the course of his experiments, Mr Delaval made use of the infusions of brazil wood, logwood, fustic, turmeric, red saunders, alkanet, lap-green, kermes, and all the other transparent coloured liquors he had tried before, among which were infusions of red and yellow flowers, without observing the least variation in the result.

Some liquors are apt to become totally opaque by evaporation; the reason of which may be the crystallization of saline matters, or the coalescence of the particles into masses, differing considerably in density from the menstrua in which they were dissolved. When this opacity takes place, our author has constantly observed, that they become incapable of entering the pores of wool, silk, or other matters of that kind, or of adhering to their surface; and consequently unfit for the purpose of dyeing. This he supposes to arise from their increased bulk; for the attractive force by which the particles cohere together is weakened in proportion as their bulk increases; so that the degree of magnitude of the colouring particles, which is essential to the opacity of liquors, is inconsistent with the minuteness requisite for dyeing. An instance of this is given in an infusion of fustic. Having infused some of this wood in such a quantity of water, that the latter was saturated with the colouring particles, he evaporated the liquor to a solid consistence with an uninterrupted, but very gentle heat. During every part of the process the liquor continued transparent, and the solid extract yielded by it transmitted a yellow colour when spread thin, but appeared black when thicker masses were viewed. Having prepared another pint of this liquor, he evaporated half the water, and allowed the remainder to become cold. In this state it became turbid and opaque; on filtering, a transparent tincture passed through, an opaque fecula remaining on the paper. This fecula did not adhere to the paper, but was easily separable from it: on being dried, it appeared white with a slight tinge of yellow; but was nevertheless soluble in water, and by solution gave a liquid in all respects similar to the original infusion.

"From these circumstances (says he) it appears that a given proportion of water, or a sufficient degree of heat, is requisite to the solution of the colouring particles of fustic. And experience evinces, that those particles which are too gross to pass through filtering paper, are incapable of entering the pores or firmly cohering to the surface of bodies. Many ingredients, such as the colouring particles of logwood, kermes, and various other matters, are soluble in water in every proportion; and therefore their infusions are not subject to become opaque or turbid during their evaporation. The solid extracts obtained by evaporation reflect no colour, but are black.

Our author also formed solid masses by mixing a small quantity of drying oil with pigments which consist chiefly of colouring matter; as Prussian blue, indigo, and lap-green. These paints likewise exhibit their respective colours only by transmitted light; appearing entirely black when viewed by reflection. Influences of blackness arising from this density of the colouring matter may be observed in several kinds of fruits, as black currants, cherries, &c., for the juices of these appear red when spread thin on a white ground, or otherwise viewed by transmitted light.

Mr Delaval's next attempt was to consider the action and properties of the colouring particles of opaque bodies themselves, and the means by which these colours are produced. Here our author endeavours to prove, that these colours of opaque bodies appear on the same principles as those already mentioned, which seem black when very dense, but show their proper tinge when spread thin upon a white ground. On this subject the following experiments were made.

1. Grasps, and other green leaves of plants, were digested in rectified spirit of wine; by which means a transparent green tincture was obtained. One of the vials formerly mentioned being filled with this liquid, it was observed to transmit a vivid green colour; but the other part of the tincture, which was contiguous to the uncovered side of the vial, reflected no light, and therefore appeared black.

2. Having poured some of the tincture into a China cup, the bottom was thereby made to look green, exactly resembling the colour which had been extracted from the leaves.

3. After the colour had been totally abstracted by the vinous spirit, the leaves remained apparently unaltered, either as to figure or texture; but were entirely white, or had their whiteness slightly tinged with brown.

4. Red, purple, and blue flowers, were also digested in spirit of wine; all of which yielded their colouring matter to the spirit, and became white by being deprived of it. From most of these flowers, however, the spirit acquired either no tinge at all, or only a very faint one; but when acidulated, it became red, and by the addition of an alkali appeared blue, purple, or green, according to the quantity of alkali and the nature of the infusion. In these states, all of them, when viewed... viewed by transmitted light, or poured upon a white ground, showed their colours, but universally appeared black by reflection.

5. Red, purple, and blue flowers, were digested in water slightly acidulated with nitrous acid. Thus, red infusions were obtained, which, by saturation with sea-salt, might be preserved for many years.

6. The same liquors were changed green, blue, or purple, by the addition of an alkali; but here the case was the same as before; all of them yielding vivid colours by transmission, but none by reflection. In making this experiment, care must be taken to add the alkali very gradually; for if too much is put in at once to the red liquor, the intermediate colours between the red and the green will be wanting. To half an ounce of the red infusion it is proper to add, at once, only the smallest quantity that can be taken up on the point of a pen; repeating this addition slowly, until each of the colours be produced.

7. The flowers, after having been repeatedly macerated in acidulated water, lost their colouring matter, and became white.

8. Yellow flowers also communicated their colours to water and to spirit of wine. The infusions and tinctures of these flowers were subjected to the same experiments as had been employed in the examination of the liquors already mentioned; and appeared yellow by transmitted light, but did not reflect any colour.

9. White paper, linen, &c. may be tinged of any of these colours, by dipping them in the infusions; and the consideration of the manner in which the colours are imparted to the linen, affords much insight into the manner in which natural colours are produced. It has already been observed, that, when the colouring matter of plants is extracted from them, the solid fibrous parts, thus deprived of their covering, display their natural whiteness. White linen, paper, &c. are formed of such fibrous vegetable matter; which is bleached by dissolving and detaching the heterogeneous colouring particles. When these are dyed or painted with vegetable colours, it is evident that they do not differ in their manner of acting on the rays of light from natural vegetable bodies; both yielding their colours by transmitting, through the transparent coloured matter, the light which is reflected from the white ground. This white matter frequently exists, without any considerable mixture, in plants, while they are in a state of vegetation; as cotton, white flowers, the pith, wood, seeds, roots, and other parts of several kinds of vegetables. When decayed trees, &c. have been long exposed to the atmosphere, their coloured juices are sometimes so perfectly extracted, that the fibres appear white. This white matter is not distinct from the vegetable earth to which plants are reduced by burning. Mr. Deleval has rendered ashes intensely white, by carefully calcining them, and afterwards grinding with a small proportion of nitre, and exposing them to such a degree of heat as would cause the nitre deflagrate with the remaining quantity of phlogiston. Lastly, the ashes were digested with marine acid, in order to dissolve the ferruginous matter diffused through them, and repeatedly washing the remainder in water. Mixing ashes thus purified with borax, and applying a vitrifying heat, an opaque enamel is obtained, remarkable for its whiteness.

Hence it appears, that the earth which forms the substance of plants is white, and separable from that which gives to each its peculiar colour; that whenever it is pure and unmixed, or diffused through in colourless media, it shows its native whiteness; and is then that the only vegetable matter endowed with a reflective power. It may be discovered, however, by other means than that of burning; thus, roses may be whitened by exposing them to the vapour of burning sulphur; an effect which cannot be attributed to the vitriolic acid, but to the phlogiston contained in that vapour. This was proved to be the case, by exposing several kinds of red and purple flowers to the phlogistic vapour issuing from hepar sulphuris; and by this every one of them was whitened; their colour being afterwards restored by the addition of an acid either mineral or vegetable.

Thus (says Mr. Delaval) it appears, that the colouring matter of the flowers is not discharged or removed, but only dissolved by the phlogiston; and thereby divided into particles too minute to exhibit any colour. In this state, together with the vegetable juice in which they are diffused, they form a colourless transparent covering, through which the white matter of the flowers is seen untinged. The colouring particles of plants consist principally of inflammable matter; and their solubility in phlogiston, and union with it, are analogous to the action of other inflammable bodies upon each other. Thus, ether dissolves all essential and expressed oils, animal empyreumatic oils, and resins. Sulphur, camphor, and almost all substances abounding in phlogiston, are soluble in oils, ardent spirits, or other inflammable menstrua. The manner in which the red colour of vegetable flowers is restored, appears to be explicable from known chemical laws. When acids are applied to the whitened flowers, they unite with the phlogiston which the sulphur had communicated, and disengage it from the colouring particles; which, being thus extricated, resume their original magnitude and hue. A change of the same kind is also produced by fixed alkali, which, like the acids, has a strong attraction for phlogiston, always changes the whitened flowers to a blue, purple, or green colour.

In like manner, the action of the rays of light operates upon coloured bodies. Thus, dyed silk, or other substances of that kind, when exposed to the sun's light, are deprived of their colour in every part on which the rays are allowed to act; whilst those preserve their colour which are defended from the light by the folds of the cloth, or intervention of any opaque body. The colours, thus impaired, may be restored if acids are applied while the injury is recent; but they are afterwards apt to fly off, on account of that volatility which is constantly imparted by inflammable matter to any other with which it is united.

Our author now proceeds, at considerable length, to prove the identity of the solar light and phlogiston; but as recent experiments have shown that these two are essentially distinct, we omit his argumentation upon this head. The error of his theory in this respect, however, does not in the least affect the doctrine concerning colours above laid down: on the contrary, the late... Latest experiments have determined, that phlogiston, in its grossest form, viz. that of common charcoal, manifests a surprising power of whitening various substances; which, according to Mr Delaval's theory, proceeds from the power it has of dissolving the colouring matter with which they are impregnated. This solvent power, according to our author, is manifest in many other instances besides those already mentioned. Silk is whitened by the phlogistic vapours of sulphur; and this operation does not appear to differ from the change effected on flowers by the same vapour. The light of the sun is found to be a necessary and essential agent in bleaching linen, wax, and various other substances; some part of the colouring matter which impairs the whiteness of these bodies not yielding to any other solvent. Red flowers are whitened by the electric spark, of whose inflammable nature we cannot entertain the least doubt; for the spark itself is a bright flame, and yields the same smell which all other phlogistic matters impart. The electric spark, in like manner, changes the blue infusion of turpentine to red (n). The effects which it produces on the turpentine, and on red flowers, do not differ from each other, except in degree only. For when vegetable matter is dissolved, it is changed from blue to red; and, when further dissolved, it is divided into particles too minute to exhibit any colour.

Solutions effected by means of phlogiston frequently are wrongly attributed to the operation of supposed acid menstrua, as several kinds of substances are capable of being dissolved indiscriminately both by acids and phlogiston. For the purpose of distinguishing, therefore, in any case between the action of the acid solvents and that of the inflammable menstrua, it is proper to examine the nature of the matter by which either of these principles are furnished. It appears from various chemical processes, that alkalies are rendered mild, and capable of crystallization, in proportion as they are united to phlogiston. The phlogisticated alkaline lixivium, when saturated, is perfectly mild; and by a slight evaporation is reduced to a concrete crystalline mass, which does not deliquesce or imbibe the least moisture from the air, and no longer retains any alkaline property. M. Beaumé, by an elegant and ingenious experiment, has proved the presence of phlogiston in mild alkalies, and has shown that their power of crystallizing depends upon their union with that principle. He heated in a silver vessel a lixivium of mild alkali, which imparted to the silver a covering or coating of inflammable matter, by which its surface was tarnished and became black. The lixivium was several times poured out of the silver vessel; and after the surface of the metal had been freed from the tarnish, the lixivium was replaced in it, and again heated, by which the tarnish was renewed; and this was repeated till the lixivium no longer communicated any stain to the silver. The causticity of the lixivium was increased in proportion as it imparted its phlogiston to the silver; and at the end of the process the alkali became perfectly caustic, and incapable of crystallizing.

Our author now goes on to prove, that fixed air is not an acid, nor a compound of air and phlogiston, as is now generally believed, but rather entirely of a phlogistic nature. For an account of his arguments in favour of this opinion, see the article Fixed Air: here we shall only consider his farther experiments on colours.

"From the preceding experiments (says he) it appears, that the colouring particles of flowers and leaves are soluble in acid, alkaline, and phlogistic menstrua. The other parts of vegetables consist of materials similar to those which are contained in their flowers and leaves, and undergo the same changes from the same causes. Having extracted from logwood its colouring particles by repeatedly boiling it in water, the wood was thus deprived of its yellow colour, and assumed a brown hue similar to that of oak-wood. Some pieces of it thus deprived of its colour were then macerated in aquafortis; and after they had undergone the action of that acid, they were washed in a sufficient quantity of water. The wood was thus reduced to whiteness."

Here our author observes, that though most authors Logwood who treat of colouring substances describe logwood as affording only a red colour, he was never able to procure any yellow other colour from it but yellow. It imparts yellow with water and orange colours to distilled water. Other waters extract a red tinge from it by means of the alkali which they contain. These observations are also applicable to the other dyeing woods, kermes, and various other articles of the materia medica. By a similar treatment, fustic wood also lost its colouring matter, and became white.

The results of all the experiments above related are, that the colouring matter of plants does not exhibit any colour by reflection, but by transmission only; that their solid earthly substance is a white matter; and that it is the only part of vegetables which is endowed with a reflective power; that the colours of vegetables are produced by the light reflected from this white matter, and transmitted from thence through the coloured coat or covering which is formed on its surface by the colouring particles; that whenever the colouring matter is either discharged or divided by solution into particles too minute to exhibit any colour, the solid earthly substance is exposed to view, and displays that whiteness which is its distinguishing characteristic.

Mr Delaval next proceeds to examine the coloured parts of animal substances, and finds them exactly similar, with regard to the manner in which the colour animal substances is produced, to the vegetable bodies already treated of. The tinctures and infusions of cochineal and of kermes yield their colours when light is transmitted through them, but show none by reflection. On diluting fresh ox-gall with water, and examining it in the phials already mentioned, that part of it which was in the neck of the phial, and viewed by transmitted light, was yellow; but the anterior surface was black, and reflected no colour. Flesh derives its colour entirely from the blood, and when deprived of it the fibres and vessels are perfectly white; as are likewise the membranes.

(n) This effect of the electric spark is now known to be produced, not by its phlogistic nature, but by the generation of an acid. branes, finews, and bones, when freed from their aqueous and volatile parts; in which case they are a mere earth, unalterable by fire, and capable of imparting an opaque whiteness to glass.

On examining blood diluted with water in one of the phials formerly described, it transmitted a red colour, and the anterior surface was almost, but not entirely, black; for it received a slight hue of brown from some coagulated particles that were suspended in the liquor. In order to procure blood sufficiently diluted, and at the same time equably and perfectly dissolved, he mixed as much erucor with spirit of sal ammoniac as imparted a bright colour to it. The liquor being then viewed in the phial, that part which was contained in the neck, and transmitted the light, appeared of a fine red; but the anterior part reflecting no light, was intensely black. Hence it appears, that the florid red colour of the flesh arises from the light which is reflected from the white fibrous substance, and transmitted back through the red transparent covering which the blood forms on every part of it.

Blood, when recently drawn, does not assume the appearance common to transparent coloured liquors; for these, when too mally to transmit light from their farther surfaces, always appear black; but blood, when recently drawn, always shows a fine red colour, in whatever way it be viewed. This is occasioned by a white matter diffused through the blood; and which is easily separated from the erucor, by dividing it after coagulation into a number of thin pieces, and washing in a sufficient quantity of pure water. Thus the water acquires a red colour, and ought to be changed daily. In a few days it will acquire no more tinge; and the remaining masses of the erucor are no longer red, but white.

In like manner, the red colour of the shells of lobsters, after boiling, is no more than a mere superficial covering spread over the white calcareous earth of which the shells are composed, and may be easily removed from the surface by scraping or filing. Before the application of heat, this superficial covering is much denser; insomuch that, in some parts of the shell, it appears quite black, being too thick to admit the passage of the light to the shell and back again; but where this transparent blue colour of the unboiled lobster is thinner, it constantly appears like a blue film. In like manner, the colours of the eggs of certain birds are entirely superficial, and may be scraped off, leaving the white calcareous earth exposed to view.

The case is the same with feathers, which owe their colours entirely to a very thin layer of some transparent matter upon a white ground. Our author afterwards obtained this by scraping off the superficial colours from certain feathers which were strong enough to bear the operation; and thus separated the coloured layers from the white ground on which they had been naturally spread. The lateral fibres of the feathers cannot indeed have their surfaces separated in this manner; but their texture, when viewed by a microscope, seems to indicate, that the colours are produced on them by no other means than those already related. In the examination of some animal subjects where the colouring matter could not be separated by chemical means, our author had recourse to mechanical division; but this can only be employed when the principal part of the white substance is unmixed with the coloured coat or covering which is spread upon its surface. All of them, however, by whatever means their colours could be separated, showed that they were produced in the same manner, namely, by the transmission of light from a white ground through a transparent coloured medium.

The coloured substances of the mineral kingdom are of the very numerous, and belong principally to two classes, viz., earths and metals. The former, when pure, are all perfectly white, and their colours arise from phlogistic or metallic mixtures. Calcareous earths, when indurated, constitute marble, and may be tinged with various colours by means of metallic solutions; all which are similar in their nature to the dyes put upon silk, cotton, or linen, and invariably proceed from the same cause, viz., the transmission of light through a very thin and transparent coloured medium. Flints are formed from siliceous earths, and owe their colour to phlogiston. When sufficiently heated, they are rendered white by the loss of the inflammable matter which produced their colour. When impregnated with metals, they form agates, cornelians, jasper, and coloured crystals. The coloured gems also receive their different hues from metals; and all of them may be imitated by glasses tinged with such phlogistic or metallic matters as enter into the composition of the original substances.

Thus our author concludes, that the coloured earths, Of metals, gems, &c., exhibit their various tints in the same manner with other substances; viz., by the transmission of light reflected from a white ground. Our author, however, proceeds farther; and affirms, that even the colours of metals themselves are produced in the same manner.

"Gold (says he) exhibits a white light, which is tinged with yellow. I have used this expression, because it appears from experiment that gold reflects a white light, and that its yellow colour is a tinge superadded to its whiteness. The experiment is thus set forth by Sir Isaac Newton. Gold in this light (that is, a beam of white light) appears of the same yellow colour as in day light; but by intercepting at the lens a due quantity of the yellow-making rays, it will appear white, like silver, as I have tried; which shows, that its yellowness arises from the excess of the intercepted rays, tinging that whiteness with their colour when they are let pass.

"I have already shown, by numerous experiments, in what manner coloured tinges are produced; and it uniformly appears, from all these experiments, that colours do not arise from reflection, but from transmission only. A solution of silver is pellucid and colourless. A solution of gold transmits yellow, but reflects no colour. This metal also, when united with glass, yields no colour by reflection, but by transmission only. All these circumstances seem to indicate, that the yellow colour of gold arises from a yellow transparent matter, which is a constituent part of that metal; that it is equally mixed with the white particles of the gold, and transmits the light which is reflected by them, in like manner as when silver is gilt, or foils are made by covering white metals with transparent colours. But these fictitious coverings are only superficial; whereas the yellow matter of gold is diffused throughout the whole substance of the metal, and..." and appears to envelope and cover each of the white particles. In whatsoever manner the yellow matter of gold is united to its white substance, it exists in a rare state; for it bears only the same proportion to the white particles of the gold as that of the yellow-making rays which were intercepted bears to all the other rays comprised in the white light of the sun.

"Sir Isaac Newton has shown, that when spaces or interfaces of bodies are replenished with media of different densities, the bodies are opaque; that those surfaces of transparent bodies reflect the greatest quantity of light which intercede media that differ most in their refractive densities; and that the reflections of very thin transparent substances are considerably stronger than those made by the same substances of a greater thickness. Hence the minute portions of air, or of the rarer medium, which occupies spaces void of other matter, reflect a vivid white light whenever their surfaces are contiguous to media whose densities differ considerably from their own; so that every small mass of air, or of the rarer medium, which fills the pores or interfaces of dense bodies, is a minute white substance. This is manifest in the whiteness of froth, and of all pellucid colourless bodies; such as glas, crystal, or salts, reduced to powder, or otherwise flawed: for in all these instances a white light is reflected from the air or rarer medium which intercede the particles of the denser substances whose interfaces they occupy."

From these principles our author takes occasion to explain the reason why the particles of metals which yield no colour by incident light, when suspended in their solvents, are disposed to exhibit colours when separated from them. Hence also we see why opaque white substances are rendered pellucid by being reduced to uniform masses whose component parts are everywhere nearly of the same density; for as all pellucid substances are rendered opaque and white by the admixture of pellucid colourless media of considerably different densities, they are again deprived of their opacity by extricating these media which kept their particles at a distance from each other; thus froth or snow, when resolved into water, lose their whiteness, and assume their former pellucid appearance. In like manner, by proper fluxes, the opaque white earths are reduced to pellucid colourless glaies; because all reflections are made at the surfaces of bodies differing in density from the ambient medium, and in the confines of equally dense media there is no reflection.

As the calces of metals are enabled to reflect their colours by the intervention of the particles of air; for, when mixed with oil in the making of paints, they always assume a darker colour, because the excess of the density of oil over that of air forms a sensible difference when comparatively considered with respect to the specific gravity of the rarer metals. From this cause perceptibly less light is reflected from the molecule of oil than from those of air, and consequently the mass appears darker. The case, however, is different with such paints as are formed of the denser metals; as vermillion, minium, &c.: for though oil differs very considerably from air in its specific density, yet it also differs very much in this respect from the denser metallic powders; and the molecule of oil which divide their particles act upon the light so strongly, that the reflection occasioned by them cannot be distinguished from those which are caused by rarer media. Hence, though we mix vermilion or minium with oil, the colour is not sensibly altered.

This part of our author's theory, however, seems liable to objection: for though it be true that the calces of some metals are denser than others, yet that is, in comparative speaking, but in a very small proportion; nor is even the difference of density between oil and the calces of the heavier metals at all comparable to that between the density of air and oil. Thus, tho' the calx of iron may be 10 or 11 times more dense than oil; yet, as the latter is between 500 and 600 times denser than air, the small difference between the oil and metallic calx ought to be imperceptible. In this respect, indeed, there are considerable differences with regard to the oils employed, which cannot be supposed to arise from the mere circumstance of density. Thus the colour of vermilion, when mixed with turpentine-varnish, is much brighter than with linseed-oil; and yet the difference between the densities of linseed-oil and turpentine-varnish is very trifling. The mere action of heat likewise has a surprizing effect in this case. Thus the red calx of iron, called scarlet ochre, by being only heated a certain degree, appears of a very dark purple, resuming its red colour when cold; and this variation may be induced as often as we please by only heating it over the fire in a shovel. In like manner, by gradually heating red lead, it may be made to assume a most beautiful crimson colour; which growing gradually darker, becomes at last almost quite black. On cooling, if the heat has not been raised too high, it gradually returns through the same shades of colour, until at last it fixes in its original hue. These immense differences in colour cannot by any means be attributed either to the expulsion of air or to an alteration in density. The fire indeed does certainly expand these calces as well as other bodies; but as the medium interposed between their particles is thus also expanded, the colour ought at least to remain the same, if not to become lighter, on account of the superior expansion of air to that of metal by the same degree of heat. It would seem, therefore, that the action of the element of fire itself has a considerable share in the production of colours; and indeed its share in the operations of nature is so great, that we might well think it strange if it should be entirely excluded from this.

With regard to semipellucid substances, which appear of one colour by incident and another by transmitted light, our author likewise endeavours to show, that no reflection is made by the coloured matter, but only by the white or colourless particles. They consist of pellucid media, throughout which white or colourless opaque particles are dispersed. The latter are disposed at such distances from each other, that some of the incident rays of light are capable of passing through the intervals which intercede them, and thus are transmitted through the semipellucid mass. Some sorts of rays penetrate through such masses, while others which differ from them in their refrangibility are reflected by the white or colourless particles; and from thence are transmitted through the pellucid part of the medium which intervenes between the reflecting particles and the anterior surface of the mass. On the same principle our author explains the blue colour of the sky, the green colour of the sea, and other natural phenomena; and from his numerous experiments on this subject at last concludes, "that the power by which the several rays of light are transmitted through different media is inherent in the particles themselves, and therefore is not confined to the surfaces of such media. For if the transmissive force was exerted at the surface only, the thinner plates of coloured substances would act upon the rays as powerfully as thicker masses. But it appears from experiment, that in proportion as the rays pass through different thicknesses of coloured media, they exhibit colours differing not only in degree, but frequently in species also.

"The sun's light, by which bodies are illuminated, consists of all the rays by which a white light is compounded. These rays, in their entire and undivided state, are incident upon the opaque particles of semipellucid substances, and upon the colouring particles of transparent-coloured substances, whenever these media are exposed to the light. When the rays accede to the opaque particles of semipellucid substances, some sorts of them are reflected back from the anterior surface of those particles; the other sorts of rays, which are not reflected back, are diverted from the direction which is opposite to the anterior surface of the opaque particles, and passing through the intervals between the particles, are transmitted through the mass.

"When the rays are incident upon the particles of transparent-coloured bodies, none of them are reflected back; because the colouring particles are not endowed with any reflective power: but some of the rays are either stopped at the anterior surface of the particles, or are diverted into such directions as render them incapable of passing towards the farther side of the mass; and consequently such rays cannot be transmitted. The rays which are not thus intercepted or dispersed, are transmitted in the same manner as those which pass through semipellucid media. Thus it is evident, that the coloured rays which are transmitted through semipellucid substances are inflected by the opaque particles; and those which are transmitted through transparent-coloured substances are inflected by the colouring particles. From the preceding observations likewise it appears, that the particles of coloured media inflect the several sorts of rays according to the several sizes and densities of the particles; also in proportion to the inflammability of the media which owe their colour to them; and it is manifest that the transmission of coloured rays depends upon their inflection. All these observations are conformable to Sir Isaac Newton's doctrine, that the rays of light are reflected, refracted, and inflected, by one and the same principle acting variously in various circumstances."

The most remarkable part of Mr Davel's doctrine is that concerning the metals; for the better understanding of which we shall premise a short abstract of his general doctrine concerning white bodies, and the manner in which light is reflected by them. "All the earths (he observes), which in their natural state are of a pure white, constitute transparent colourless media when vitrified with proper fluxes, or when dissolved in colourless menstrua; and the saline masses obtainable from their solutions are transparent and colourless while they retain the water which is essential to their crystallization, and are not flawed or reduced to powder; but after their pores and interstices are opened in such a manner as to admit the air, they become then white and opaque by the entrance of that rare medium. The earthy particles which form the solid parts of bodies generally exceed the others in density; consequently these particles, when contiguous to the rare media already mentioned, must reflect the rays of light with a force proportionate to their density. The reflective power of bodies does not depend merely upon their excess of density, but upon their difference of density with respect to the surrounding media. Transparent colourless particles, whose density is greatly inferior to that of the media they come between, also powerfully reflect all sorts of rays, and thereby become white. Of this kind are the air or other rare fluids which occupy the interstices of liquors; and in general of all denser media into whose interstices such rare particles are admitted.

"Hence we may conclude, that white opaque bodies are constituted by the union or contiguity of two or more transparent colourless media differing considerably from each other in their reflective powers. Of these substances we have examples in froth, emulsions, or other imperfect combinations of pellucid liquors, milk, snow, calcined or pulverized salts, glaas or crystal reduced to powder, white earths, paper, linen, and even those metals which are called white by mineralogists and chemists: for the metals just mentioned do not appear white unless their surfaces be rough; as in that case only there are interstices on their surface sufficient to admit the air, and thus make a reflection of a white and vivid light.

"But the polished surfaces of metallic mirrors reflect the incident rays equably and regularly, according to their several angles of incidence; so that the reflected rays do not interfere with each other, but remain separate and unmixed, and therefore distinctly exhibit their several colours. Hence it is evident, that white surfaces cannot act upon the light as mirrors; because all the rays which are reflected from them are blended in a promiscuous and disorderly manner.

"The above-mentioned phenomena give much insight into the nature and cause of opacity; as they clearly show, that even the rarest transparent colourless substances, when their surfaces are adjacent to media differing greatly from them in refractive power, may thereby acquire a perfect opacity, and may assume a rependency and hue so similar to that of white metals, that the rarer pellucid substances cannot by the sight be distinguished from the dense opaque metals. And this similarity to the surfaces of metals occurs in the rare pellucid substances, not only when, from the roughness of their surfaces, they resemble unpolished metals in whiteness, but also when, from their smoothness, they resemble the polished surfaces of metals.

"Metals seem to consist entirely of transparent matter, and to derive their apparent opacity and lustre solely from the copious reflection of light from their surfaces. The analogy between the metals and transparent media, as far as respects their optical properties, will appear from the following considerations.

1. All metals dissolved in their proper menstrua are transparent. 2. By the union of two or more transparent media, substances are constituted which are similar to metals in their opacity and lustre, as plumbago and marcasites. 3. The transparent substances of metals, as well as those of minerals, by their union with phlogiston, acquire their strong reflective powers from which their lustre and opacity arise. 4. The surfaces of pellucid media, such as glass or water, assume a metallic appearance, when by their smoothness, difference of density with respect to the contiguous media, or any other cause, they are disposed copiously to reflect the light.

"From all these considerations it is evident, that opaque substances are constituted by the union or contiguity of transparent colourless media, differing from one another in their reflective powers; and that, when the common surface, which comes between such media, is plane, equal, and smooth, it reflects the incident rays equally and regularly as a mirror; but when the surface is rough and unequal, or divided into minute particles, it reflects the incident rays irregularly and promiscuously in different directions, and consequently appears white."

From all these experiments we can only conclude, that the theory of colours seems not yet to be determined with certainty; and very formidable, perhaps unanswerable, objections, might be brought against every hypothesis on this subject that hath been invented. The discoveries of Sir Isaac Newton, however, are sufficient to justify the following

APHORISMS.

1. All the colours in nature proceed from the rays of light. 2. There are seven primary colours; which are red, orange, yellow, green, blue, indigo, and violet. 3. Every ray of light may be separated into the seven primary colours. 4. The rays of light in passing through the same medium have different degrees of refrangibility. 5. The difference in the colours of light arises from its different refrangibility: that which is the least refrangible producing red; and that which is the most refrangible violet. 6. By compounding any two of the primary colours, as red and yellow, or yellow and blue, the intermediate colour, as orange or green, may be produced. 7. The colours of bodies arise from their dispositions to reflect one sort of rays, and to absorb the other: those that reflect the least refrangible rays appearing red; and those that reflect the most refrangible, violet. 8. Such bodies as reflect two or more sorts of rays appear of various colours. 9. The whiteness of bodies arises from their disposition to reflect all the rays of light promiscuously. 10. The blackness of bodies proceeds from their incapacity to reflect any of the rays of light (c).

(c) From hence it arises, that black bodies, when exposed to the sun, become sooner heated than all others.

Entertaining Experiments, founded on the preceding Principles.

I. Out of a single colourless ray of light to produce seven other rays, which shall paint, on a white body, the seven primary colours of nature.

Procure of an optician a large glass prism DEF, plate well polished, two of whose sides must contain an angle of about sixty-four degrees. Make a room quite dark, and in the window shutter AB, cut a round hole, about one-third of an inch in diameter, at C, through which a ray of light LI passing, falls on the prism DEF: by that it is refracted out of the direction LT, in which it would have proceeded into another GH; and, falling on the paper MNSX, will there form an oblong spectrum PQ, whose ends will be semicircular, and its sides straight; and if the distance of the prism from the paper be about eighteen feet, it will be ten inches long, and two inches wide. This spectrum will exhibit all the primary colours: the rays between P and V, which are the most refracted, will paint a deep violet; those between V and I, indigo; those between I and B, blue; those between B and G, green; those between G and Y, yellow; those between Y and O, orange; and those between O and R, being the least refracted, an intense red. The colours between these spaces will not be everywhere equally intense, but will incline to the neighbouring colour: thus the part of the orange next to R, will incline to a red; that next to Y, to a yellow: and so of the rest.

II. From two or more of the primary colours, to compose others that shall, in appearance, resemble those of the former.

By mixing the two homogeneous colours red and yellow, an orange will be produced, similar in appearance to that in the series of primary colours; but the light of the one being homogeneous, and that of the other heterogeneous, if the former be viewed through a prism it will remain unaltered, but the other will be resolved into its component colours red and yellow. In like manner other contiguous homogeneous colours may compound new colours; as by mixing yellow and green, a colour between them is formed; and if blue be added, there will appear a green that is the middle colour of those three. For the yellow and blue, if they are equal in quantity, will draw the intermediate green equally toward them, and keep it, as it were, in equilibrium, that it verge not more to the one than to the other. To this compound green there may be added some red and violet; and yet the green will not immediately cease, but grow less vivid; till by adding more red and violet it will become more diluted; and at last, by the prevalence of the added colours, it will be overcome, and turned into some anomalous colour.

If the sun's white, composed of all kinds of rays, be added... added to any homogeneal colour, that colour will not vanish, nor change its species, but be diluted; and by adding more white, it will become continually more diluted. Lastly, if red and violet be mixed, there will be generated, according to their various proportions, various purples, such as are not like, in appearance, to the colour of any homogeneal light; and of these purples, mixed with blue and yellow, other new colours may be composed.

III. Out of three of the primary colours, red, yellow, and blue, to produce all the other prismatic colours, and all that are intermediate to them.

Provide three panes of glass of about five inches square; and divide each of them, by parallel lines, into five equal parts. Take three sheets of very thin paper; which you must paint, lightly, one blue, another yellow, and the third red (p). Then paste on one of the glasses five pieces of the red paper; one of which must cover the whole glass, the second only the four lower divisions, the third the three lower, the fourth the two lowest, and the fifth the last division only. On the other two glasses five pieces of the blue and yellow papers must be pasted in like manner. You must also have a box of about six inches long, and the same depth and width as the glasses; it must be black on the inside; let one end be quite open, and in the opposite end there must be a hole large enough to see the glasses completely. It must also open at the top, that the glasses may be placed in it conveniently.

When you have put any one of these glasses in the box, and the open end is turned toward the sun, you will see five distinct shades of the colour it contains. If you place the blue and yellow glasses together, in a similar direction, you will see five shades of green distinctly formed. When the blue and red glasses are placed, a bright violet will be produced; and by the red and yellow, the several shades of orange.

If, instead of placing these glasses in a similar position, you place the side AB of the yellow glass against the side BD of the blue, you will see all the various greens that are produced by nature (e); if the blue and red glasses be placed in that manner, you will have all the possible varieties of purples, violets, &c.; and, lastly, if the red and orange glasses be so placed, there will be all the intermediate colours, as the marigold, aurora, &c.

IV. By means of the three primary colours, red, yellow, and blue, together with light and shade, to produce all the gradations of the prismatic colours.

On seven square panes of glass, paste papers that are painted with the seven prismatic colours, in the same manner as in the last experiment. The colours for the orange, green, indigo, and violet, may be made by mixing the other three. Then with bitre (f), well diluted, shade a sheet of very thin paper, by laying it light on both its sides. With pieces of this paper cover four-fifths of a glass, of the same size with the others, by laying one piece on the four lowest divisions, another on the three lowest, a third on the two lowest, and the fourth on the lowest division only, and leaving the top division quite uncovered. When one of the coloured glasses is placed in the box, together with the glasses of shades, so that the side AB of the one be applied to the side BC of the other, as in fig. 3, the several gradations of colours will appear shaded in the same manner as a drapery judiciously painted with that colour.

It is on this principle that certain French artists have proceeded in their endeavours to imitate, by designs printed in colours, paintings in oil: which they do by four plates of the same size, on each of which is engraved the same design. One of these contains all the shades that are to be represented, and which are painted either black or with a dark grey. One of the three other plates is coloured with blue, another with red, and the third with yellow; each of them being engraved in those parts only which are to represent that colour (g); and the engraving is either stronger or weaker, in proportion to the tone of colour that is to be represented (h).

These four plates are then passed alternately under the press, and the mixture of their colours produces a print that bears no small resemblance to a painting. It must be confessed, however, that what has been hitherto done of this kind falls far short of that degree of perfection of which this art appears susceptible. If they who engrave the best in the manner of the crayon were to apply themselves to this art, there is reason to expect

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(p) Water-colours must be used for this purpose: the blue may be that of Prussia, and very bright; the red, carmine; and the yellow, gamboge, mixed with a little saffron. These colours must be laid very light and even, on both sides of the paper.

(e) In the first position of the glasses, the quantity of blue and yellow being equal, the same sort of green was constantly visible: but by thus inverting the glasses, the quantity of the colours being constantly unequal, a very pleasing variety of tints is produced.

(f) The bitre here used must be made of root, not that in stone.

(g) When a red drapery is required, it is engraved on the plate assigned to that colour; and so of yellow and blue: but if one of the other colours be wanting, suppose violet, it must be engraved on those that print the red and blue: and so of the rest. The plates of this kind have been hitherto engraved in the manner of mezzotinto; but these, unless they are skilfully managed, soon become smutty. Engravings in the manner of the crayon will perhaps answer better.

(h) The principal difficulty in this sort of engraving arises from a want of a skilful management, in giving each plate that precise degree of engraving which will produce the tone of colour required. If a bright green is to be represented, there should be an equal quantity of graving on the red and yellow plates: but if an olive green, the yellow plate should be engraved much deeper than the red. VI. To make figures appear of different colours successively.

Make a hole in the window-shutter of a dark room, through which a broad beam of light may pass, that is to be refracted by the large glass prism ABC, which may be made of pieces of mirrors cemented together, and filled with water. Provide another prism DEF, made of three pieces of wood; through the middle of this there must pass an axis on which it is to revolve. This prism must be covered with white paper; and each of its sides cut through in several places, so as to represent different figures, and those of each side should likewise be different. The inside of this prism is to be hollow, and made quite black, that it may not reflect any of the light that passes through the sides into it. When this prism is placed near to that of glass, as in the figure, with one of its sides EF perpendicular to the ray of light, the figures on that side will appear perfectly white; but when it comes into the position gh, the figures will appear yellow and red; and when it is in the position kl, they will appear blue and violet. As the prism is turned round its axis, the other sides will have a similar appearance. If instead of a prism a four or five sided figure be here used, the appearances will be still further diversified.

This phenomenon arises from the different refrangibility of the rays of light. For when the side EF is in the position gh, it is more strongly illuminated by the least refrangible rays; and wherever they are predominant, the object will appear red or yellow. But when it is on the position kl, the more refrangible rays being then predominant, it will appear tinged with blue and violet.

VI. The solar magic lantern.

Procure a box, of about a foot high, and eighteen inches wide, or such other similar dimensions as you shall think fit; and about three inches deep. Two of the opposite sides of this box must be quite open; and in each of the other sides let there be a groove, wide enough to pass a stiff paper or pasteboard. This box must be fastened against a window on which the sun’s rays fall direct. The rest of the window should be closed up, that no light may enter. Provide several sheets of stiff paper, which must be blacked on one side. On these papers cut out such figures as you shall think proper; and placing them alternately in the grooves of the box, with their blacked sides towards you, look at them through a large and clear glass prism; and if the light be strong, they will appear to be painted with the most lively colours in nature. If you cut on one of these papers the form of the rainbow, about three quarters of an inch wide, you will have a lively representation of that in the atmosphere.

This experiment may be further diversified, by pasting very thin papers, lightly painted with different colours, over some of the parts that are cut out; which will appear to change their colours when viewed through the prism, and to stand out from the paper, at different distances, according to the different degrees of refrangibility of the colours with which they are painted. For greater convenience, the prism may be placed in a stand on a table, at the height of your eye, and made to turn round on an axis, that when you have got an agreeable prospect, you may fix it in that position.

VII. The prismatic camera obscura.

Make two holes F, f, in the shutter of a dark chamber, near to each other; and against each hole place a prism ABC, and abe, in a perpendicular direction, that their spectrums NM may be cast on the paper in a horizontal line, and coincide with each other; the red and violet of the one being in the same part with those of the other. The paper should be placed at such a distance from the prisms that the spectrum may be sufficiently dilated. Provide several papers nearly of the same dimensions with the spectrum, crop these papers, and draw lines parallel to the divisions of the colours. In these divisions cut out such figures as you shall find will have an agreeable effect, as flowers, trees, animals, &c. When you have placed one of these papers in its proper position, hang a black cloth or paper behind it, that none of the rays that pass through may be reflected and confuse the phenomenon. The figures cut on the paper will then appear strongly illuminated with all the original colours of nature. If while one of the prisms remains at rest, the other be revolved on its axis, the continual alteration of the colours will afford a pleasing variety; which may be further increased by turning the prism round in different directions.

When the prisms are so placed that the two spectrums become coincident in an inverted order of their colours, the red end of one falling on the violet end of the other; if they be then viewed through a third prism DH, held parallel to their length, they will no longer appear coincident, but in the form of two distinct spectrums, pt and nm (fig. 6.), crossing one another in the middle, like the letter X: the red of one spectrum and the violet of the other, which were coincident at NM, being parted from each other by a greater refraction of the violet to p and m, than that of the red to n and t.

This experiment may be further diversified by adding two other prisms, that shall form a spectrum in the same line, and contiguous to the other; by which not only the variety of figures, but the vicissitude of colours, will be considerably augmented.

VIII. The diatonic scale of colours.

The illustrious Newton, in the course of his investigations of the properties of light, discovered that the length of the spaces which the seven primary colours possess in the spectrum, exactly corresponds to those of chords that sound the seven notes in the diatonic scale of music. As is evident by the following experiment.

On a paper in a dark chamber, let a ray of light be largely refracted into the spectrum AFTMGP, and mark the precise boundaries of the several colours, as a, b, c, &c. Draw lines from those points perpendicular to the opposite side, and you will find that the spaces M r f F, by which the red is bounded; r g e f, by which the orange is bounded; g p e d, by which the yellow is bounded, &c., will be in exact proportion. to the divisions of a musical chord for the notes of an octave; that is, as the intervals of these numbers \(1, \frac{3}{4}, \frac{2}{3}, \frac{3}{5}, \frac{2}{7}, \frac{3}{8}\).

IX. Colorific music.

Father Castel, a Frenchman, in a curious book he has published on chromatics, supposes the note ut to answer to blue in the prismatic colours; the note re to yellow, and mi to red. The other tones he refers to the intermediate colours; from whence he constructs the following gamut of colorific music:

| Ut | Blue | |--------|---------------| | Ut sharp | Sea-green | | Re | Bright green | | Re sharp | Olive green | | Mi | Yellow | | Fa | Aurora | | Fa sharp | Orange | | Sol | Red | | Sol sharp | Crimson | | La | Violet | | La sharp | Blue Violet | | Si | Sky blue | | Ut | Blue |

This gamut, according to his plan, is to be continued in the same manner for the following octave; except that the colours are to be more vivid.

He supposes that these colours, by striking the eye in the same succession as the sounds (to which he makes them analogous) do the ear, and in the same order of time, they will produce a correspondent sensation of pleasure in the mind. It is on these general principles, which F. Castel has dilucidated in his treatise, that he has endeavoured, though with little success, to establish his ocular harpsichord.

The construction of this instrument, as here explained, will show that the effects produced by colours by no means answer those of sounds, and that the principal relation there is between them consists in the duration of the time that they respectively affect the senses.

Between two circles of pasteboard, of ten inches diameter, AB and CD, inclose a hollow pasteboard cylinder E, 18 inches long. Divide this cylinder into spaces half an inch wide, by a spiral line that runs round it from top to bottom, and divide its surface into six equal parts by parallel lines drawn between its two extremities; as is expressed in the figure.

Let the circle AB, at top, be open; and let that at bottom, CD, be closed, and supported by an axis or screw, of half an inch diameter, which must turn freely in a nut placed at the bottom of a box we shall presently describe. To the axis just mentioned adjust a wooden wheel G, of two inches and a half in diameter, and that has 12 or 15 teeth, which take the endless screw H. Let this cylinder be inclosed in a box ILMN (fig. 9.) whose base is square, and at whose bottom there is a nut in which the axis F turns. Observe that the endless screw H should come out of the box, that it may receive the handle O, by which the cylinder is to be turned.

This box being closed all round, place over it a tin covering A, which must be perforated in different parts; from this cover there must hang three or four lights, so placed that they may strongly illumine the inside of the cylinder. In one side of this box (which should be covered with pasteboard) cut eight apertures, \(a, b, c, d, e, f, g, h\), of half an inch wide, and \(i\) of an inch high; they must be directly over each other, and the distance between them must be exactly two inches. It is by these openings, which here correspond to the musical notes, that the various colours analogous to them are to appear; and which being placed on the pasteboard cylinder, as we have shown, are reflected by means of the lights placed within it.

It is easy to conceive, that when the handle O is turned, the cylinder in consequence rising half an inch, if it be turned five times round, it will successively show, at the openings made in the side of the box, all those that are in the cylinder itself, and which are ranged according to the direction of the inclined lines drawn on it. It is therefore according to the duration of the notes which are to be expressed, that the apertures on the cylinder are to be cut. Observe, that the space between two of the parallel lines drawn vertically on the cylinder, is equal to one measure of time; therefore, for every turn of the cylinder, there are six measures, and thirty measures for the air that is to be played by this instrument.

The several apertures being made in the side of the cylinder, in conformity to the notes of the tune that is to be expressed, they are to be covered with double pieces of very thin paper, painted on both sides with the colours that are to represent the musical notes.

This experiment might be executed in a different manner, and with a much greater extent; but as the entertainment would not equal the trouble and expense, we have thought it sufficient to give the above piece, by which the reader will be enabled to judge how far the analogy supposed by F. Castel really exists.