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 hypothesis 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 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 was caused by the refraction 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 the 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 confirmed 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 homogeneous 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 foot 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 thinness 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 thickness of their component particles. He also contrived 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 the colours of thin substances, found that the same 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 refracts or reflects 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 quickly covered over on the convex side. Then, holding a quire of white paper at the centre of the sphere to which the speculum 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 others in the same parts. 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 invisible 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 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 | Red | | Greenish-blue | Red | | Red | 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 reflexion 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 Huygens Delaval, Mr Delaval's experimental Inquiry into the cause of the various changes of colours in opaque and coloured bodies. He 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 least 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 tinged it yellow. When we meet with authors who mention a blue or greenish colour communicated by silver, the same must have been, that the silver used in such processes was mixed with copper. Mr Delaval affirms us, from his own experience, that silver purified by the test 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 glasses 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 colour, 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.

There 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 attributes 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 thickness 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 Mr Canton, I was agreeably surprized to find, that he had not experimented 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 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 place 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 insane author (Dr Priestley), whom we have just now begun to argue 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 inquiries, particularly those of the Abbe Mazzei. This gentleman, endeavouring to give a very high polish to the flat side of an object-glass, happened to be rubbing it against another piece of flat 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 Abbe observed, were in proportion 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 reflection, 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 reflection 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 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 compelled 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 Plane Glasses | Order of the Colours in Newton's Object Glasses | |--------------------------------------|-----------------------------------------------| | Order I | Black | | | Whitish oval | | | Yellow border | | | Deep purple | | | Blue | | | Orange | | | Purple | | | Greenish blue | | | Yellowish green | | | Purplish red | | Order II | Black | | | Blue | | | White | | | Yellow | | | Red | | | Violet | | | 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 glasses 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 even 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 Abbe, in order to verify his hypothesis, tried the experiment on thicker substances. He put between his glasses a little ball of fuel, 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 fuel; but, though he rubbed them together as before, and used other solid substances besides fuel, his endeavours to produce the colours had no effect. But, rubbing them with more violence in a circular manner, he was surprised, 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 Newtonian there must be some error in Newton's hypothesis, by hypothesising, 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 fuel 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 fuel.

"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 fuel, 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 Haukbee'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 fuel; and indeed he could perceive but little Little difference between the colours of fuel, 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 fuel 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 a 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 dissipate 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 infusor itself between the glasses, the colours grew fainter, and the rings were contracted; and ascribing these colours to the thicknesses 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 fines of the refractions of these mediums.

"The Abbé Mazeas, 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 anything to them, it served rather to modify than to generate them.

"M. du Tour gave particular attention to the preceding observations of the Abbé Mazeas. He repeated the experiments 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 adhesion of the air to their surfaces; and that mere pressure is not sufficient to expel it; except, as the Abbé Mazeas 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 glass. 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, serves, 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 he could account for the colours exhibited 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 thicknesses 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 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 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 six 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 six 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 tallow tanned with milk and water. The frame that supported the piece of tallow 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 tallow from the mirror, so that they were very large when the tallow 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 tallow, 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 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 a line, 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 plateboard 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 passage out from between the solid and impenetrable 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 11½ 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 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 anything 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. Klinge-gerlerna) 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. Klinge-gerlerna, being communicated to Mr Dollond by Mr 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 stopped 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.

"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 shewing the produced 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 infected 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 assures 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..." convenience 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 feasible 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 misapprehension 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, 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 dispersing power, its errors would have been perfectly corrected."

From all these experiments we can only conclude Theory of that the theory of colours seems not yet to be determined with certainty; and very formidable, perhaps uncertain, 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.

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.

Process of an optician a large glass prism DEF, Pl. lxviii 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 LL passing, falls on the prism DEF: by that it is refracted out of the direction IT, 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 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 dilated; 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 kind of rays, be added to any homogeneous 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 homogeneous 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*. 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†; 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 marmagold, 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 primitive 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 bister‡, 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 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.

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* You must use water-colours 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.

† In the first position of the glasses the quantity of blue and yellow being equal, the same fort 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.

‡ The bister here used must be made of root, not that in flour. 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*, and the engraving is either stronger or weaker, in proportion to the tone of colour that is to be represented†.

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 they would produce far more finished pieces than we have hitherto seen.

V. 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 g h, the figures will appear yellow and red; and when it is in the position k l, 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 g h, 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 k l, 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 18 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 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 a b c, 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, cross 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,

* 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 finnity. Engravings in the manner of the crayon, will perhaps answer better.

† The principal difficulty in this sort of engraving arises from 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. 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, p t, and n m, (fig. 6.) cradling 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 vivacity 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 found 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 AFTMGF, 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, 2, 3, 4, 5, 6, 7, 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 primitive 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. Cattel has elucidated 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, so 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 ½ 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 Chronicle piece, by which the reader will be enabled to judge how far the analogy supposed by F. Castel really exists.