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 ἀπό, per, "through," and ὄψις, ὤψ, "see."
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. Present 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. by passing out of air into plate the glass BGC, are turned from their natural course into XCH, 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. 3. 2. A double convex lens, as B. 3. A plano-concave lens, that is, plane on one side... 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, 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, 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 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. When an object is viewed through a glass whose two surfaces are parallel, it will appear of its natural dimensions; its situation only being a small matter altered, in proportion to the thickness of the glass, and the obliquity of the rays.
4. All the rays of light, whether diverging, parallel, or converging, that fall on a single or double convex lens, will meet in a focus behind the glass; and the distance of that focus will be greatest in diverging, and least in converging, rays.
5. When parallel or converging rays fall on a single or double concave lens, they will diverge behind it. If they be diverging at their incidence, they will become more so by passing through it.
6. When an object is viewed thro' two convex lenses, its apparent length, or diameter, will be to its real length, as the distance of the focus of the object-glass is to that of the eye-glass.
By these, and the foregoing 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, 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 thro' 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 window of a room fix three small pieces of paper, as A, B, C, at the height of your eye; 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 rays that come from these 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. It will at first appear cut in, as to the naked eye; but if you continue to observe it some time, without changing 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 changing either 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 discernable by all persons.
V. The 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 pasteboard, 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 (a), 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 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.
VI. 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 foot telescope into the scioptic ball, and turn it about till it be directly opposite to the sun (d). Then place the paste-
(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 it 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 it, 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 such erect position.
(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 objects 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 of the window, as in Experiment V, the lens will answer the purpose at any time. board, 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 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 thro' a lens whose focus is at six 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.
VII. 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 flutter be thrown on a large concave mirror, properly fixed in a frame. Then take a slip or thin plate of glass; and sticking 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.
VIII. 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 one that may have some advantages not to be found in others, and which appears to be the invention of M. Guyot.
Let ABCD be a frame of wood, of two feet long and about 20 inches wide; let its four sides be two inches and a half thick, and firmly joined together. In a groove formed in this frame place a plate of clear glass, E; and if the upper side of the glass were convex, it would be still better. To each of the corners of this frame join a leg, with a hinge, that it may turn up under the table. To the under part of the frame join four pieces of light wood, as H, which must also have hinges to fold up; and observe, that when they are let down, as in the figure, they must closely join, by means of hooks, it being quite necessary that no light enter the box. For this reason, the inside of the box should be lined with black cloth.
To that just described, there must be added a smaller box M, in which must be an inclined mirror N, and in one of its sides a moveable tube O, five or six inches long. This tube must be furnished with a convex glass, the focus of which, by the reflection of the mirror, must reach the glass E in the frame. There must also be a covering of black stuff, in form of a tent, to place over the top of the frame, by means of four little poles that go into holes in the corners of it. There must be an opening to this tent on the side A B, by a curtain to be drawn up; and which you are to let down over you, when you place yourself under it; that no light may enter. The three other sides should hang down some inches over the frame.—This camera is, indeed, something more cumbersome than those that have been hitherto invented; and yet, if properly made, it will not weigh more than from 20 to 25 pounds. On the other hand, it is much more convenient; for as the coloured rays of objects paint themselves on the bottom of the glass in the frame, you may draw them without having your hand between the rays and their image.
When you have placed the frame on a spot a little elevated, that nothing may intercept the rays from falling on the glass in the tube, you fix a sheet of transparent varnished paper on the glass in the frame, by means of wax at its corners. Then placing yourself under the curtain, you trace on the paper all the outlines of the objects there represented; and if you think fit, you may also mark the extent of the shadows. If you want only the outlines, you may lay a thin plate of glass on that in the frame, and trace the strokes with a pencil and carmine. After which you must dip a sheet of paper in water, without making it too wet; and spreading it lightly over that glass, you will have the impression of the design there drawn.
Note. By each of these methods you will have the objects either in their natural position, or reversed; which will be an advantage when the design is to be engraved, and you would have it then appear in the natural position. In using this machine, you should make choice of those objects on which the sun then shines, as the appearance of the shadows add greatly to the beauty of the design. There are, however, circumstances in which it is to be avoided, as when you would paint a rising or setting sun, &c.
IX. 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 be a tin box, eight inches high, ten long, and six wide (or any other similar dimensions). At the top must be a funnel E, of four inches in diameter, with a cover F, which, at the same time that it gives a passage to the smoke, prevents the light from coming out of the box.
On the side A C there is a door, by which is adjusted a concave mirror G, of metal or tin, and of five inches diameter; being part of a sphere whose diameter is 18 inches. This mirror must be so disposed that it may be pushed forward or drawn back by means of the handle H, that enters the tin tube I, which is foldered to the door. In the middle of the box must be placed a low tin lamp K, which is to be moveable. It should have three or four lights, that must be at the height of the focus of the mirror G. In the side BD, and opposite to the mirror, there must be an aperture of three inches wide and two inches and a half high; in which is to be fixed a convex glass L, of the same dimension, whose focus must be from four inches and a half to five inches, so that the lamp may be placed both in its focus, and in that of the concave mirror. On the same side is to be placed a piece of tin MN, of four inches and a half square, having an opening at the sides of about four inches and a half high, and a quarter of an inch wide. Through this opening or groove are to pass the glasses, on which are painted the figures that are to be seen on the cloth. In this tin piece, and opposite to the glass L, let there be an aperture of three inches and a quarter long, and two inches and a quarter high; to which must be adjusted a tube O, of the same form, and six inches long. This tube is to be fixed into the piece MN. Another tube, six inches long, and moveable, must enter that just mentioned, in which must be placed two convex lenses, P and Q; that of P may have a focus of about three inches; and that of Q, which is to be placed at the extremity of the tube, one of 10 or 12 inches. The distance between these glasses is to be regulated by their foci. Between these glasses there must be placed a pasteboard R, in which is an aperture of an inch wide, and 4-5ths of an inch high. By placing this tube farther in or out of the other, the images on the cloth will appear larger or smaller.
From what has been said of the preceding machines, the construction of this will be easily understood. The foci of the concave mirror, and the lens L, meeting in the flame of the lamp, they together throw a strong light on the figures painted on the glasses that pass through the groove MN, and by that means render their colours distinct on the cloth. The rays from those glasses passing through the lens P are collected by the aperture in the pasteboard R, and conveyed to the lens Q, by which they are thrown on the cloth.
The lantern being thus adjusted, you must provide plates of clear glass, of 12 or 15 inches long, and three inches wide, which are to be placed in thin frames, that they may pass freely through the groove MN, after being painted in the manner we shall now describe.
**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 those colours that are used. When the outlines are dry, you colour the figures with their proper tints or gradations. Transparent colours are most proper for this purpose, such as carmine, lake, Prussian blue, verdigris, &c., and these must 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 this lantern is lighted, and, by drawing out the tube to a proper length, the figures painted on the glasses 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 (s), which every one may perform according to his own taste; either by movements 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.
**X. 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 groove MN, at the same time, (fig. 7.)
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 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 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.
**XI. 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 AB, of 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 six inches long, and half an inch wide. At the bottom of this box there must be a door
---
(s) There are in the Philosophical Essays of M. Moucheronbroek, different methods of performing all these various movements, by some mechanical contrivances that are not difficult to execute. 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 appear 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.
XII. To produce the appearance of a Phantom, upon a pedestal placed on the middle of a table.
Inclosc a common small magic lantern in a box ABCD, 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 not making. This aperture should likewise be oval, and of a size adapted to the cone of light that is to pass thro' 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 inclosed in a painted tin box of about a foot high, and with an aperture at top something like fig. 10. 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 the vertical groove ab. 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 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 surprize, 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. Tho' we have mentioned a small magic lantern, yet the whole apparatus may be so enlarged, that the phantom may appear of a formidable size.
XIII. The Magical Theatre.
By making some few additions to the magic lantern with the square tube, used in Experiment ix. 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, a foot Fig. 12. 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 six 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 six 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 (r). Inclose these grooves between two convex rectangular glasses, of six 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 groves, 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
(r) In the decorations, the clouds and the palaces of the gods should descend; earthly palaces, gardens, &c. enter at the sides. On this paper are to be exhibited the images of all those objects, that, by passing successively through the grooves, are to represent a theatric 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 theatric 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 further 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, 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 grooves 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 (c). 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 same 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 Eneas bearing his father Anchises, followed by his son Ascanius, and some Trojans. With this glass may be represented the flight of the Trojans, and the embarkment of Eneas; 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 Eneas sailing for Italy.—On the first glass must be painted the sea, as in the tenth Exper. 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 Eneas 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
(c) He that moves the glasses, seeing the effect they produce, is the better able to render the representation as natural as possible.
mands 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 are 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.
DIO