THAT part of OPTICS which treats of the laws of refraction, and the effects which the refraction of light has in vision. The word is originally Greek, formed of δίος, per, "through," and ὀψικαῖ, Isee.
As this and the other branches of OPTICS are fully treated under the collective name, we shall here, 1. Just give a summary of the general principles of the branch, in a few plain aphorisms, with some preliminary definitions; and, 2. Prefect our readers with a set of entertaining experiments, illustrative of, or dependent upon, those principles.
DEFINITIONS.
1. When a ray of light passing out of one medium into another of a different density, is turned from that straight line in which it would otherwise proceed into one of a different direction, it is said to be refracted. Thus the rays AB, AC, &c. (fig. 1.) by passing out of air into the glass BGC, are turned from their natural course into that of BF, CF, &c. and are therefore said to be refracted by the lens BGC.
2. Any spherical transparent glass, that converges or diverges the rays of light as they pass through it, is called a lens.
3. Of lenses there are five sorts: 1. A plane or single convex lens, which is plane on one side and convex on the other; as AZ, fig. 2. 2. A double convex lens, as B. 3. A plano-concave lens, that is, plane on one side and concave on the other, as C. 4. A double concave, as D. And, 5. A meniscus, which is convex on one side and concave on the other, as E.
4. The point C. (fig. 2.) round which the spherical surface of a lens, as AZ, is described, is called its centre; the line XY, drawn from that centre perpendicular to its two surfaces, is the axis; and the point V, to which the axis is drawn, is the vertex of that lens.
5. When the rays of light that pass through a single or double convex lens are brought into their smallest compass, that point is the focus of the lens.
6. In optical instruments, that lens which is next the object is called the object glass; and that next the eye, the eye glass.
7. The distance between the line AB, (fig. 3.) and the perpendicular EF, is called the angle of incidence; and the distance between the line BD and the perpendicular EF, is called the angle of refraction.
APHORISMS.
1. A ray of light passing obliquely out of one medium into another that is denser, will be refracted toward the perpendicular; as the ray AB, by passing out of air into glass, is refracted into BF, inclined to the perpendicular AF. On the contrary, a ray passing out of a denser into a rarer medium, will be refracted from the perpendicular; as the ray BC, passing out of the glass GH into air, is refracted into DI.
2. The sines of the angles of incidence and refraction, when the lines that contain them are all equal, will have a determinate proportion to each other, in the same mediums; which, between air and water, will be as 4 to 3; between air and glass, as 3 to 2, nearly; and in other mediums, in proportion to their densities.
3. Any object viewed through a glass, whose two surfaces are parallel, will appear of its natural shape and dimensions, provided it be only of the size of the pupil of the eye, and the light proceeding from it be received directly through the glass by one eye only. In all other situations an alteration will be perceived not only in its apparent situation, but its dimensions also. This alteration will be greater in proportion to the thickness of the glass, and the obliquity of the rays; in general, it is so small as to be overlooked.
4. All the rays of light which fall upon a convex lens, whether parallel, converging, or diverging to a certain degree, will be made to meet in a focus on the other side: but if they diverge excessively, they will not do so. Thus, if rays diverge from a point placed before the glass, at the focal distance from it, they will become parallel after passing through it; and if the point from which they proceed be nearer the glass than its focal distance, they will still continue to diverge, though in a less degree than before.
5. When parallel rays fall upon a concave lens, they will be made to diverge after passing through it. If they are diverging already before they fall upon the glass, they will diverge more after passing through it; or even if they are converging to a certain degree, they will diverge upon passing through a concave lens; but if the convergence is very great, they will converge after passing through the glass, though to a more distant point than that at which they would otherwise have met.
6. When an object is viewed through two convex lenses, its apparent diameter ought to be to its real one as the distance of the focus of the object glass is to that of the eye glass; but by reason of the aberration of the rays of light, the magnifying power will be somewhat greater or less in proportion to the diameter of the object.
By these aphorisms we are enabled to account for the various effects of dioptric machines, as refracting telescopes, microscopes, the camera obscura, &c. See Optics.
ENTERTAINING EXPERIMENTS.
I. Optical Illusions.
On the bottom of the vessel ABCD, (fig. 4.) place three pieces of money, as a shilling, a half crown, and crown; the first at E, the second at F, and the last at G. Then place a person at H, where he can see no further into the vessel than I: and tell him, that by pouring water into the vessel you will make him see three different pieces of money: bidding him observe carefully whether any money goes in with the water.
Here you must observe to pour in the water very gently, or contrive to fix the pieces, that they may not move out of their places by its agitation.
When the water comes up to K, the piece at E will become visible; when it comes up to L, the pieces at E and F will appear; and when it rises to M, all the three pieces will be visible.
From what has been said of the refraction of light, the cause of this phenomenon will be evident: for while the vessel is empty, the ray HI will naturally proceed in a straight line: but in proportion as it becomes immersed in water, it will be necessarily refracted into the several directions, NE, OF, PG, and consequently the several pieces must become visible.
II. Optical Augmentation.
Take a large drinking glass of a conical figure, that is small at bottom and wide at top; in which put a shilling, and fill the glass about half full with water: then place a plate on the top of it, and turn it quickly over, that the water may not get out. You will then see on the plate, a piece of the size of a half crown; and somewhat higher up, another piece of the size of a shilling.
This phenomenon arises from seeing the piece through the conical surface of the water at the side of the glass, and through the flat surface at the top of the water, at the same time: for the conical surface dilates the rays, and makes the piece appear larger; but by the flat surface the rays are only refracted, by which the piece is seen higher up in the glass, but still of its natural size. That this is the cause will be further evident by filling the glass with water; for as the shilling cannot be then seen from the top, the large piece only will be visible.
III. Optical Subtraction.
Against the wainscot of a room fix three small pieces of paper, as A, B, C, (fig. 5:) at the height of your eye; Fig. 5: and placing yourself directly before them, shut your right eye and look at them with the left; when you will see only two of those papers, suppose A and B: but altering the position of your eye, you will then see the third and one of the first, suppose A; and by altering your position a second time, you will see B and C; but never all three of them together.
The cause of this phenomenon is, that one of the three pencils of ray- that come from those objects, falls constantly on the optic nerve at D: whereas to produce distinct vision, it is necessary that the rays of light fall on some part of the retina E, F, G, H. We see by this experiment, one of the uses of having two eyes; for he that has one only, can never see three objects placed in this position, nor all the parts of one object of the same extent, without altering the situation of his eye.
IV. Alternate Illusion.
With a convex lens of about an inch focus, look attentively at a silver seal, on which a cipher is engraved. graved. It will at first appear cut in, as to the naked eye; but if you continue to observe it some time, without changing your situation, it will seem to be in relief, and the lights and shades will appear the same as they did before. If you regard it with the same attention still longer, it will again appear to be engraved: and so on alternately.
If you look off the seal for a few moments, when you view it again, instead of seeing it, as at first, engraved, it will appear in relief. If, while you are turned toward the light, you suddenly incline the seal, while you continue to regard it, those parts that seemed to be engraved will immediately appear in relief; and if, when you are regarding these seeming prominent parts, you turn yourself so that the light may fall on the right hand, you will see the shadows on the same side from whence the light comes, which will appear not a little extraordinary. In like manner the shadows will appear on the left, if the light fall on that side. If, instead of a seal, you look at a piece of money, these alterations will not be visible, in whatever situation you place yourself.
It has been suspected that this illusion arises from the situation of the light: and, in fact, "I have observed (says M. Guyot, from whom this article is taken), that when I have viewed it with a candle on the right, it has appeared engraved; but by changing the light to the left side, it has immediately appeared in relief." It still, however, remains to be explained, why we see it alternately hollow and prominent, without either changing the situation or the light. Perhaps it is in the light itself that we must look for the cause of this phenomenon; and this seems the more probable, as all these appearances are not discernible by all persons.
Mr. William Jones of Holborn, has remarked to us, that this illusion is still more extraordinary and permanent, when you look at a cavity in a seal or other object through the three eyes glases of a common four glas refracting telescope: all cavities viewed through these glases appear constantly relief, in almost all situations of the light you see them with.
V. The Dioptrical Paradox.
A new and curious optical, or what may be called properly a dioptrical, deception, has been made by Mr W. Jones. Its effect is, that a print, or an ornamented drawing, with any object, such as an ace of diamonds, &c., in the centre F, (fig. 6.) will be seen as the ace of clubs when it is placed in the machine ABDC, and viewed through a single glass only contained in the tube E. The construction of this machine is truly simple.
The glass in the tube F, which brings about this surprising change, is somewhat on the principle of the common multiplying glass, as represented at G, which by the number of its inclined surfaces, and from the refractive power of the rays proceeding from the objects placed before it, shows it in a multiplied state or quantity. Its only difference is, that the sides of this glass are flat, and diverge upwards from the base to a point in the axis of the glass like a cone: the number of the sides is fix; and each side, from its angular position to the eye, has the property of refracting from the border of the print F such a portion of it (designedly there placed), as will make a part in the composition of the figure to be represented: for the hexagonal and conical figure of this glass prevents any light of the ace of diamonds in the centre being seen; consequently the ace of clubs being previously and mechanically drawn in the circle of refraction in fix different parts of the border, at 1, 2, 3, 4, 5, 6, and artfully disguised in the ornamental border by blending them with it, the glass in the tube at E will change the appearance of the ace of diamonds F into the ace of clubs G. In the same manner may other prints undergo similar changes, according to the will of an ingenious draughtman who may design them. The figure of the glass is clearly shown at H.
VI. Camera Obscura, or Dark Chamber.
Make a circular hole in the shutter of a window, from whence there is a prospect of the fields, or any other object not too near; and in this hole place a convex glass, either double or single, whose focus is at the distance of five or six feet (A). Take care that no light enter the room but by this glass: at a distance, from it equal to that of its focus, place a paletteboard, covered with the whitest paper; which should have a black border, to prevent any of the side rays from disturbing the picture. Let it be two feet and a half long, and 18 or 20 inches high: bend the length of it inwards, to the form of part of a circle, whose diameter is equal to double the focal distance of the glass. Then fix it on a frame of the same figure, and put it on a moveable foot, that it may be easily fixed at that exact distance from the glass where the objects paint themselves to the greatest perfection. When it is thus placed, all the objects that are in the front of the window will be painted on the paper, in an inverted position (B), with the greatest regularity and in the most natural colours.
If you place a moveable mirror without the window; by turning it more or less, you will have on the paper
(A) The distance should not be less than three feet; for if it be, the images will be too small, and there will not be sufficient room for the spectators to stand conveniently. On the other hand, the focus should never be more than 15 or 20 feet, for then the images will be obscure, and the colouring faint. The best distance is from 6 to 12 feet.
(B) This inverted position of the images may be deemed an imperfection, but is easily remedied: for if you stand above the board on which they are received, and look down on it, they will appear in their natural position; or if you stand before, and placing a common mirror against your breast in an oblique direction, look down in it, you will there see the images erect, and they will receive an additional lustre from the reflection of the glass; or place two lenses, in a tube that draws out; or, lastly, if you place a large concave mirror at a proper distance before the picture, it will appear before the mirror, in the air, and in an erect position. paper all the objects that are on each side of the window (c).
If instead of placing the mirror without the window you place it in the room, and above the hole (which must then be made near the top of the shutter), you may receive the representation on a paper placed horizontally on a table; and draw, at your leisure, all the objects that are there painted.
Nothing can be more pleasing than this experiment, especially when the objects are strongly enlightened by the sun: and not only land prospects, but a sea-port, when the water is somewhat agitated, or at the setting of the sun, presents a very delightful appearance.
This representation affords the most perfect model for painters, as well for the tone of colours, as that degradation of shades, occasioned by the interposition of the air, which has been so justly expressed by some modern painters.
It is necessary that the paper have a circular form; for otherwise, when the centre of it was in the focus of the glass, the two sides would be beyond it, and consequently the images would be confused. If the frame were contrived of a spherical figure, and the glass were in its centre, the representation would be still more accurate. If the object without be at the distance of twice the focal length of the glass, the image in the room will be of the same magnitude with the object.
The lights, shades, and colours, in the camera obscura, appear not only just, but, by the images being reduced to a smaller compass, much stronger than in nature. Add to this, that these pictures exceed all others, by representing the motion of the several objects; thus we see the animals walk, run, or fly; the clouds float in the air; the leaves quiver; the waves roll, &c.; and all in strict conformity to the laws of nature. The best situation for a dark chamber is directly north, and the best time of the day is noon.
VII. To show the Spots on the Sun's Disk, by its image in the Camera Obscura.
Put the object glass of a 10 or 12 feet telescope into the scioptic ball, and turn it about till it be directly opposite to the sun (d). Then place the pasteboard, mentioned in the last experiment, in the focus of the lens; and you will see a clear bright image of the sun, of about an inch in diameter, in which the spots on the sun's surface will be exactly described.
As this image is too bright to be seen with pleasure by the naked eye, you may view it through a lens whose focus is at fix or eight inches distance; which at the same time that it prevents the light from being offensive, will, by magnifying both the image and the spots, make them appear to greater advantage.
VIII. To magnify small Objects by means of the Sun's Rays let into a Dark Chamber.
Let the rays of light that pass through the lens in the shutter be thrown on a large concave mirror, properly fixed in a frame. Then take a flip or thin plate of glass; and flicking any small object on it, hold it in the incident rays, at a little more than the focal distance from the mirror; and you will see on the opposite wall, amidst the reflected rays, the image of that object, very large, and extremely clear and bright. This experiment never fails to give the spectator the highest satisfaction.
IX. The Portable Camera Obscura.
The great pleasure produced by the camera obscura in the common form, has excited several to render it more universally useful by making it portable, easily fixed on any spot, and adapted to every prospect. We shall not here examine the merits of the various sorts that have been invented; but content ourselves with describing of two late improved constructions, as made and sold by the opticians of the present time, and that appear in their construction the most convenient and advantageous of any yet contrived.
The pocket or portable camera obscura, with a drawer to draw out in the front, is represented in fig. 7. Fig. 7. The images of the objects before the instrument are reflected upon a glass ground rough on its upper side, and that is placed at top of the hinder part of the box, under the moveable cover represented in the figure. The images represented therein will afford a most beautiful and perfect piece of perspective or landscape of whatever is before the camera, and more particularly so if the sun shines upon the objects. The outlines of them may easily be traced on the glass by a black lead pencil. There is sometimes a scale of proportions placed in the upper surface of the drawer, by which any particular building or other object may be drawn in a given proportion or magnitude, and according to the figures inserted on the scale, which are adapted to the focus or foci of the lenses made use of in the camera. The glasses that are made use of in this camera are only three, and are represented in fig. 8. The con- Fig. 8. vex glass A is placed in the front of the drawer of the camera, and is of a focus agreeable to the length of the box. The mirror CE reclines in the box in an angle of 45 degrees from a perpendicular situation. The rays flowing from the object F through the convex glass A to the plane mirror CE will be reflected from it, and meet in points on the glass placed horizontally in the direction CD, and will form thereon the aforementioned images. If on this glass and oiled paper or any other transparent substance be placed the images
(c) There is another method of making the dark chamber; which is by a scioptic ball, that is, a ball of wood, through which a hole is made, in which hole a lens is fixed: this ball is placed in a wooden frame, in which it turns freely round. The frame is fixed to the hole in the shutter: and the ball, by turning about, answers, in great part, the use of the mirror, on the outside of the window. If the hole in the window be no bigger than a pea, the object will be represented without any lens, though by no means so distinctly, or with such vivid colours.
(d) When the sun is directly opposite to the hole, the lens will itself be sufficient: or by means of the mirror on the outside window, as in Experiment VI. the lens will answer the purpose at any time. images will be clearly represented, and sufficiently so to delineate them by a black lead pencil or crayon. Instead of the glass CD, or sometimes underneath it, is often placed a double convex lens of a focus somewhat shorter than the length of the box; this alteration considerably brightens the appearance of the images, and renders them as vivid as the objects themselves, though not quite so accurate in their contours or outlines as by the preceding method.
Another kind of portable camera obscura is, where the images are formed upon white paper, and the several parts of the camera fold up out of a box shaped like a book or chest. This way of the images being formed on paper is a much preferable one to the preceding method, and admits of their being traced on the paper with the utmost readiness. This instrument, as open out of its case and ready for use, is represented in fig. 9. The front and sides fold up to the height of about two feet from the case EFG, by means of hinges placed at PH, &c. The head of ABCD, about five inches square and high, containing the mirror L and the convex lens beneath it, fits on at CD, and the inner square tube of it is moved up and down by rack-work and a pinion NM. This motion serves to adjust the convex lens d to its proper focal distance from the white paper placed within side at the bottom of the box EFG, so that the images may be formed with the greatest possible distinctness. In tracing these images the face is applied close to the hole in the front at K, and the hand in the sleeve in the front at the bottom of FG. When the sides and front are unhooked and folded down, they all lie close in the box EFG, and the lid O folds down as a top on them close, and the box remains then the size of a common folio book; and is covered with calf leather and lettered on the back in perfect imitation of one.
By the diagonal position of a plane mirror the curious opera glass is constructed, by which any person may be viewed in a theatre or public company, and yet know nothing of it. It consists only in placing a concave glass near the plane mirror, in the end of a short round tube, and a convex glass in a hole in the side of the tube. Then holding the end of the tube with the glass to the eye, all objects next to the hole in the side will be reflected so as to appear in a direct line forward, or in a position at right angles to the person's situation who is looked at. Plane glasses instead of a concave and convex may be used; but in this case there will be no magnitude of the object, but it will appear brighter. It is called by opticians the diagonal opera glass.
X. The Magic Lantern.
This very remarkable machine, which is now known over all the world, caused great astonishment at its origin. It is still beheld with pleasing admiration; and the spectator very frequently contents himself with wondering at its effects, without endeavouring to investigate their cause. The invention of this ingenious illusion is attributed to the celebrated P. Kircher, who has published on various sciences, works equally learned, curious, and entertaining. Its design is to represent at large, on a cloth or board, placed in the dark, the images of small objects, painted with transparent colours on plates of glass.
The construction is as follows: Let ABCD (fig. 10.) Fig. 10. be the side of a tin box, eight inches high, eight inches long, and ten broad (or any other similar dimensions), the top of which must have a funnel, with a cover, as represented in fig. 11.; which at the same time it gives a Fig. 11. passage to the smoke, prevents the light from coming out of the box. In the middle of the bottom of the box must be placed a low tin lamp E, which is to be moveable. It should have three or four lights, that must be at the height of the centre of the glasses in the tubes N and O. In the largest of these tubes must be placed a glass semiglobular lens N, about four inches diameter; and in the smaller one a double convex lens o, about 2 1/2 inches diameter, and fix inches focus, the length of the tubes holding them about 4 1/2 inches each; the inner tube containing the small lens o must be a sliding one, in order to adjust it at a proper distance from the painted sliders, so that the objects thereon may be distinctly represented on the cloth or white wall. A slit or opening between the glass N and the front side BGDH of the box must be made large enough to admit the sliders to be passed through, (as in fig. 11.). The clearness of the light, and the objects upon the cloth, will depend much upon the light of the lamp; it will therefore be proved best, to place, instead of the common lamp E, a kind of the new or Argand's patent lamp, which will be found considerably to improve the effect of the lantern by its superior strength of light.
From the construction of this lantern it is evident that when the glass sliders, with the painted figures, are placed in the groove or slit in the lantern for that purpose, and the room darkened, a quantity of light from the lamp at E will be collected by the lens N, and refracted upon the cloth placed opposite, and that by moving the sliding tube containing the small lens o gradually in or out as occasion may require, this lens will form images of the figures on the sliders in their distinct colours and proportions, with the appearance of life itself, and of any size from six inches to seven feet, according to the distance of the lantern from the cloth. The lantern, with one of the sliders ready for use, is clearly represented in fig. 11. By the aid of the new patent lamp aforementioned, considerable useful improvements are made to the lantern. Mr Jones, optician, of Holborn, has contrived an apparatus to be applied to it, that converts it into a microscope by night, and it shows all the variety of transparent and many of the opaque objects magnified upon a cloth or screen opposite, similar to the figures above mentioned, but not in so large a degree; about one or two feet diameter is the utmost that can at present be obtained.
Method of painting the Glasses for the lantern. Draw on a paper the subject you desire to paint, and fix it at each end to the glass. Provide a varnish with which you have mixed some black paint; and with a fine pencil draw on the other side of the glass, with very light touches, the design drawn on the paper. If you are desirous of making the painting as perfect as possible, you should draw some of the outlines in their proper colours, provided they are the strongest tints of these colours that are used. When the outlines are dry, you colour the figures with their proper tints or degradations... degradations. Transparent colours are most proper for this purpose, such as carmine, lake, Prussian blue, verdigrise, &c. and these may be tempered with a strong white varnish, to prevent their peeling off. You are then to shade them with black mixed with the same varnish, or with bistre, as you find convenient. You may also leave strong lights in some parts, without any colours, in order to produce a more striking effect. Observe, in particular, not to use more than four or five colours, such as blue, red, green, and yellow. You should employ, however, a great variety of tints, to give your painting a more natural air; without which they will represent vulgar objects, which are by no means the more pleasing because they are gaudy.
When the lamp in the lantern is lighted, and, by drawing out the tube to a proper length, the figures painted on the glass appear bright and well defined, the spectator cannot fail of being highly entertained by the succession of natural or grotesque figures that are painted on the glasses. This piece of optics may be rendered much more amusing, and at the same time more marvellous, by preparing figures to which different natural motions may be given (e), which every one may perform according to his own taste; either by movement in the figures themselves, or by painting the subject on two glasses, and passing them at the same time through the groove, as will be seen in the next experiment.
XI. To represent a Tempest by the Magic Lantern.
Provide two plates of glass, whose frames are so thin that they may both pass freely through the slit or groove of the common magic lantern at the same time.
On one of these glasses you are to paint the appearance of the sea from the slightest agitation to the most violent commotion. Representing from A to B (fig. 12.) a calm; from B to C a small agitation, with some clouds; and so on to F and G, which should exhibit a furious storm. Observe, that these representations are not to be distinct, but run into each other, that they may form a natural gradation; remember also, that great part of the effect depends on the perfection of the painting, and the picturesque appearance of the design.
On the other glass (fig. 13.) you are to paint vessels of different forms and dimensions, and in different directions, together with the appearance of clouds in the tempestuous parts.
You are then to pass the glass slowly through the groove; and when you come to that part where the storm begins, you are to move the glass gently up and down, which will give it the appearance of a sea that begins to be agitated; and so increase the motion till you come to the height of the storm. At the same time you are to introduce the other glass with the ships, and moving that in like manner, you will have a natural representation of the sea, and of ships in a calm and in a storm. As you draw the glasses slowly back, the tempest will seem to subside, the sky grow clear, and the ships glide gently over the waves.—By means of two glasses disposed in this manner you may likewise represent a battle or sea-fight, and numberless other subjects, that every one will contrive according to his own taste. They may also be made to represent some remarkable or ludicrous action between different persons, and many other amusements that a lively imagination will easily suggest.
XII. The Nebulous Magic Lantern.
The light of the magic lantern, and the colour of images, may not only be painted on a cloth, but also reflected by a cloud of smoke.
Provide a box of wood or pasteboard (fig. 14.) of fig. 14. about four feet high, and of seven or eight inches square at bottom, but diminishing as it ascends, so that its aperture at top is but fix inches long, and half an inch wide. At the bottom of this box there must be a door that shuts quite close, by which you are to place in the box a chafing-dish with hot coals, on which is to be thrown incense, whose smoke goes out in a cloud at the top of the box. It is on this cloud that you are to throw the light that comes out of the lantern, and which you bring into a smaller compass by drawing out the moveable tube. The common figures will here serve. It is remarkable in this representation, that the motion of the smoke does not at all change the figures; which appears so conspicuous, that the spectator thinks he can grasp them with his hand.
Note. In this experiment some of the rays passing through the smoke, the representation will be much less vivid than on the cloth; and if care be not taken to reduce the light to its smallest focus, it will be still more imperfect.
XIII. To produce the Appearance of a Phantom upon a Pedestal placed on the middle of a Table.
Enclose a common small magic lantern in a box ABCD (fig. 15.) that is large enough to contain also an inclined mirror M, which must be moveable, that it may reflect the cone of light thrown on it by the lantern, in such a manner that it may pass out at the aperture made in the top of the box. There should be a flap with hinges to cover the opening, that the inside of the box may not be seen when the experiment is making. This aperture should likewise be oval, and of a size adapted to the cone of light that is to pass through it. There must be holes made in that part of the box which is over the lantern, to let out the smoke: and over that part must be placed a chafing-dish of an oblong figure, and large enough to hold several lighted coals. This chafing-dish may be enclosed in a painted tin box of about a foot high, and with an aperture at top something like fig. 14. It should stand on four short feet, to give room for the smoke of the lamp to pass out. There must also be a glass that will ascend and descend at pleasure in a vertical groove a b. To this glass let there be fixed a cord, that, going over a pulley c, passes out of the box at the side CD, by which
(e) There are in the Philosophical Essays of M. Muschenbroek, different methods of performing all these various movements, by some mechanical contrivances that are not difficult to execute. the glass may be drawn up, and will descend by its own weight. On this glass may be painted a spectre, or any other more pleasing figure. Observe, that the figures must be contracted in drawing, as the cloud of smoke does not cut the cone of light at right angles, and therefore the figures will appear longer than they do on the glass.
After you have lighted the lamp in the lantern, and put the mirror in a proper direction, you place the box or pedestal ABCD on a table; and putting the chafing dish in it, throw some incense in powder on the coals. You then open a trap-door, and let down the glass slowly; and, when you perceive the smoke diminish, you draw up the glass, that the figure may disappear, and shut the trap-door. This appearance will occasion no small surprise, as the spectre will seem to rise gradually out of the pedestal, and on drawing up the glass will disappear in an instant. Observe, that when you exhibit this experiment, you must put out all the lights in the room; and the box should be placed on a high table, that the spectators may not perceive the aperture by which the light comes out. Though we have mentioned a small magic lantern, yet the whole apparatus may be so enlarged, that the phantom may appear of a formidable size.
XIV. The Magic Theatre.
By making some few additions to the magic lantern with the square tube, used in Experiment X. various scenes, characters, and decorations of a theatre, may be represented in a lively manner. In this experiment it is quite necessary to make the lantern much larger than common, that the objects painted on the glasses, being of a larger size, may be represented with greater precision, and consequently their several characters more strongly marked.
Let there be made a wooden box ABCD, fig. 16, a foot and a half long, 15 inches high, and 10 wide. Let it be placed on a stand EF, that must go round it, and by which it may be fixed with two screws to a table. Place over it a tin cover, as in the common lantern. Make an opening in its two narrowest sides; in one of which place the tube H, and in the other the tube I: let each of them be fix inches wide, and five inches high: in each of these tubes place another that is moveable, in order to bring the glasses, or concave mirror, that are contained in them, to a proper distance. In the middle of the bottom of this box place a tin lamp M; which must be moveable in a groove, that it may be placed at a proper distance with regard to the glasses and mirror; this lamp should have five or fix lights, each of them about an inch long. At the beginning of the tube H, toward the part N, make an opening of an inch wide, which must cross it laterally: another of three quarters of an inch, that must cross it vertically, and be nearer the box than the first; and a third of half an inch, that must be before the first. The opening made laterally must have three or four grooves, the second two, and the third one: that different subjects of figures and decorations may be passed, either sidewise, ascending or descending, so that the scenes of a theatre may be the more exactly imitated (F). Enclose these grooves between two convex rectangular glasses, of fix inches long, and five inches high, and of about 20 inches focus; one of which must be placed at O, and the other toward P. Have another tube Q, of about a foot long, which must enter that marked H; and at its outward extremity place a lens of about 15 inches focus. There must also be a third tube R, four inches long, into which that marked I is to enter; to the exterior end of this adjust a concave mirror, whose focus must be at seven or eight inches from its reflecting surface.
The magic lantern being thus adjusted, nothing more is necessary than to provide glasses, painted with such subjects as you would represent, according to the grooves they are to enter. The lamp is then to be lighted; and placing a glass in one of the grooves, you draw out the moveable tubes till the object paints itself on a cloth to the most advantage: by which you determine the distance of the lantern and the size of the image. You then make a hole in the partition of that size, and fix in it a plate of clear glass, over which you paste a very thin paper, which must be varnished, that it may be as transparent as possible.
On this paper are to be exhibited the images of all those objects, that by passing successively through the grooves, are to represent a theatrical entertainment. The exhibition will be very agreeable; because the magic lantern being concealed behind the partition, the cause of the illusion cannot by any means be discovered.
In order to show more clearly in what manner a subject of this sort should be painted, and the glasses disposed, we will here make choice of the siege of Troy for a theatrical subject; in which will be found all the incidents necessary to the exhibition of any other subject whatever.—In the first act the theatre may represent, on one side, the ramparts of Troy; toward the back part, the Grecian camp; and at a farther distance, the sea, and the isle of Tenedos. We will suppose the time to be that when the Greeks feigned to raise the siege; and embarked, leaving behind them the wooden horse, in which were contained the Grecian soldiers.—On a glass, therefore, of the same width with the aperture made in the side AC of the box, you are to paint a deep blue curtain, lightly charged with ornaments, quite transparent. This glass is to be placed in the first vertical groove; so that by letting it gently down, its image may appear to rise in the same manner as the curtain of a theatre. All the glasses that are to ascend or descend must be bordered with thin pieces of wood, and so exactly fill the grooves, that they may not slide down of themselves.—You must have several glasses of a proper size to pass through the horizontal grooves, and of different lengths according to the extent of the subject. You may paint
(F) In the decorations, the clouds and the palaces of the gods should descend; caves and infernal palaces should ascend; earthly palaces, gardens, &c. enter at the sides. on the first, the walls of Troy. On the second, the Grecian camp. On the third, the sea, the isle of Tenedos, and a serene sky. On the fourth, the Grecian troops by detached figures. On the fifth, other troops, disposed in battalions, and placed at a distance. On the sixth, divers vessels, which as the glass advances in the groove diminish in size. On the seventh, the wooden horse and Sinon. On the eighth, Trojan men and women.
These glasses being properly painted, you place in the horizontal groove the first, second, third, and fourth. Then draw up the curtain, by letting down the glass on which it is painted, and draw away gently the fourth glass, and after that the second; then advance very gently the fifth that represents the embarkment, and pass it quite through. Next pass, the opposite way, the sixth, which represents the Grecian fleet. The objects painted on the fourth, fifth, and sixth, quite disappearing, you are to advance the seventh, on which is painted the wooden horse; and at the same time the eighth, where the Trojans will appear to draw the horse into the city. The curtain is then to be let down, that you may withdraw the scenes of the first act, and place in the grooves those that are to compose the second.—In the second act may be represented the interior part of the city of Troy; on one side may be seen the wooden horse, and in the back part the temple of Pallas. The glasses for this act may be painted in the following manner. On the first may be palaces and houses, representing the inside of a city. On the second the temple of Pallas in the centre, with a clear night and the moon. In the front may be seen the wooden horse, that the Trojans have placed near the temple of Pallas. On the third, a troop of Greeks, with Sinon at their head, who are going to open the gates of the city to the Grecians. On the fourth, different troops of armed Greeks, painted on a long glass, to afford variety. On the fifth, several troops of Trojans. On the sixth, various appearances of fire and smoke, so disposed, that this glass being drawn up above the others, the objects painted on the first glass may appear in a conflagration.
Before you draw up the curtain, you should place the first and second glasses. You then pass the whole third glass slowly; a little after, the fourth, on which are painted the different bodies of armed Greeks; and at the same time, from the opposite side, the fifth glass, that represents the Trojan troops; observing to move them slowly both in advancing and retreating, to imitate a combat (g). Then draw up, by degrees, the sixth, on which are painted the fire, flame, and smoke, so that the palaces and houses painted on the first glass may appear to take fire gradually, and at last present a general conflagration. After having represented these incidents with the greatest attention, you let fall the curtain to prepare for the third act. In this may be represented the inside of Priam's palace; where is seen an altar, round which several Trojan princesses appear, who have fled thither for safety. On the first glass may be painted the palace. On the second, a view of the back part of the palace, with the altar. On the third, Priam with several Trojan men and women. On the fourth, Pyrrhus and a troop of Greeks. On the fifth, the fame actors, with the palace in flames. On the sixth, a conflagration.—The two first glasses which are to be drawn up, should be placed before you raise the curtain. Then pass the third; next advance the fourth; which being drawn up, discovers on the fifth the palace in flames; then drawing up the sixth, let down the first, that the palace may appear entirely destroyed by the conflagration.
The fourth act may represent the environs of Troy, with a distant prospect of the sea. The first and third glasses of the first act may be here used; to which may be added a third, representing Æneas bearing his father Anchises, followed by his son Iulus and some Trojans. With this glass may be represented the flight of the Trojans; and the embarkment of Æneas with another glass, on which are painted certain vessels.—To this act the following scenes may be added: The cave of Æolus; the back part of the cave; Æolus; the winds; Juno in her chariot.
The fifth act should represent the open sea, with the fleet of Æneas sailing for Italy. On the first glass must be painted the sea, as in the eleventh experiment, or else the waves should be imitated by another glass under the first. On the second, the Trojan fleet. On the third, Neptune in his car. On the fourth, the palace of Jupiter. On the fifth, the inside of the palace; the gods assembled in council, with Venus obtaining leave of Jupiter for Æneas to land in Italy.—After having placed the first glass, that represents a calm sea, the curtain is raised, and the second scene is advanced, which contains the Trojan fleet. The first is then brought forward, to represent a violent tempest: then raising the third glass, Neptune appears, who commands the waves to be still, which is done by making the tempest subside by degrees. The fleet then advances, and passes over the whole theatre; presently after the fourth and fifth scenes descend, that represent Olympus, and finish the exhibition.
Note, We must here repeat, that if you would represent a subject of this sort to advantage, it is quite necessary that the glasses be well painted; and those that are to be in front should be in stronger and more opaque colours, that the images of those behind may not appear mixed with them, which will be the case if they are all equally transparent. The glasses should also be of different lengths; that some being placed before the others and drawn away, their extremities may not be perceived.
The larger these subjects are represented, the better effect they will have: the front of the theatre should appear to be about three feet wide; and if some parts of the figures were moveable, it would still add to the variety of the entertainment.