THE PRISM.

The prism is a triangular solid of gla.s.s, and by it the young optician may decompose a ray of light into its primitive and supplementary colours, for a ray of light is of a compound nature. By the prism the ray A is divided into its three primitive colours, blue, red, and yellow; and their four supplementary ones, violet, indigo, green, and orange. The best way to perform this experiment is to cut a small slit in a window-shutter, on which the sun shines at some period of the day, and directly opposite the hole place a prism P; a beam of light in pa.s.sing through it will then be decomposed, and if let fall upon a sheet of white paper, or against a white wall, the seven colours of the rainbow will be observed.

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COMPOSITION OF LIGHT.

The beam of light pa.s.sing through the prism is decomposed, and the s.p.a.ces occupied by the colours are in the following proportions:--red, 6; orange, 4; yellow, 7; green, 8; blue, 8; indigo, 6; violet, 11. Now, if you paste a sheet of white paper on a circular piece of board about six inches in diameter, and divide it with a pencil into fifty parts, and paint colours in them in the proportions given above, painting them dark in the centre parts, and gradually fainter at the edges, till they blend with the one adjoining. If the board be then fixed to an axle, and made to revolve quickly, the colours will no longer appear separate and distinct, but becoming gradually less visible they will ultimately appear _white_, giving this appearance to the whole surface of the paper.

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A NATURAL CAMERA OBSCURA.

The human eye is a camera obscura, for on the back of it on the retina every object in a landscape is beautifully depicted in miniature. This may be proved by the

BULLOCK'S EYE EXPERIMENT.

Procure a fresh bullock's eye from the butcher, and carefully thin the outer coat of it behind: take care not to cut it, for if this should be done the vitreous humour will escape, and the experiment cannot be performed. Having so prepared the eye, if the pupil of it be directed to any bright objects, they will appear distinctly delineated on the back part precisely as objects appear in the instrument we are about to describe. The effect will be heightened if the eye is viewed in a dark room with a small hole in the shutter, but in every case the appearance will be very striking.

THE CAMERA OBSCURA.

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This is a very pleasing and instructive optical apparatus, and may be purchased for four or five shillings. But it may be easily made by the young optician. Procure an oblong box, about two feet long, twelve inches wide, and eight high. In one end of this a tube must be fitted containing a lens, and be made to slide backwards and forwards so as to suit the focus. Within the box should be a plain mirror reclining backwards from the tube at an angle of forty-five degrees. At the top of the box is a square of unpolished gla.s.s, upon which from beneath the picture will be thrown, and may be seen by raising the lid A. To use the camera place the tube with the lens on it opposite to the object, and having adjusted the focus, the image will be thrown upon the ground-gla.s.s as above stated, where it may be easily copied by a pencil or in colours.

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The form of a camera obscura used in a public exhibition is as follows:--D D is a large wooden box stained black in the inside, and capable of containing from one to eight persons. A B is a sliding piece, having a sloping mirror C, and a double convex lens F, which may with the mirror C be slid up or down so as to accommodate the lens to near and distant objects. When the rays proceeding from an object without fall upon the mirror, they are reflected upon the lens F, and brought to fall on the bottom of the box, or upon a table placed horizontally to receive them, which may be seen by the spectator whose eye is at E.

THE CAMERA LUCIDA.

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This instrument consists of a gla.s.s prism, C, D, D, E, having four sides covered. The sides C, D, being exposed to the object to be delineated, rays pa.s.s through the gla.s.s and fall on the sloping side D, E; from this they are reflected to the top, and finally pa.s.s out of the prism to the eye;[11] now from the direction at which the rays enter the eye, it receives them as if coming from an image at A, B, and if a sheet of paper be placed below the instrument, a perfect delineation of the object may be traced with a pencil. This is a very useful instrument to young draughtsmen.

[11] The eye is to be applied to the little circular hole seen on the upper surface.

THE MAGIC LANTERN.

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This is one of the most pleasing of all optical instruments, and it is used to produce enlarged pictures of objects, which being painted on a gla.s.s in various colours are thrown upon a screen or white sheet placed against the wall of a large room. It consists of a sort of tin-box, within which is a lamp, the light of which (strongly reflected by the reflector T,) pa.s.ses through a great plano-convex lens E fixed in the front. This strongly illuminates the objects which are painted on the slides or slips of gla.s.s, and placed before the lens in an inverted position, and the rays pa.s.sing through them and the lens F, fall on a sheet, or other white surface, placed to receive the image. The gla.s.ses on which the figures are drawn are inverted, in order that the images of them may be erect.

PAINTING THE SLIDES.

The slides containing the objects usually shown in a magic lantern, are to be bought at opticians with the lantern, and can be procured cheaper and better in this way than by any attempt at manufacturing them.

Should, however, the young optician wish to make a few slides of objects of particular interest to himself, he may proceed as follows:--

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Draw first on paper the figures you wish to paint, lay it on the table, and cover it over with a piece of gla.s.s of the above shape; now draw the outlines with a fine camel's hair pencil in black paint mixed with varnish, and when this is dry, fill up the other parts with the proper colours, shading with bistre also mixed with varnish. The transparent colours are alone to be used in this kind of painting.

TO EXHIBIT THE MAGIC LANTERN.

The room for the exhibition ought to be large, and of an oblong shape.

At one end of it suspend a large sheet so as to cover the whole of the wall. The company being all seated, darken the room, and placing the lantern with its tube in the direction of the sheet, introduce one of the slides into the slit, taking care to invert the figures; then adjust the focus of the gla.s.ses in the tube by drawing it in or out as required, and a perfect representation of the object will appear.

EFFECTS OF THE MAGIC LANTERN.

Most extraordinary effects may be produced by means of the magic lantern; one of the most effective of which is a

TEMPEST AT SEA.

This is effected by having two slides painted, one with the tempest as approaching on one side, and continuing in intensity till it reaches the other. Another slide has ships painted on it, and while the lantern is in use, that containing the ships is dexterously drawn before the other, and represents _ships in the storm_.

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The effects of sunrise, moonlight, starlight, &c., may be imitated, also by means of double slides, and figures may be introduced sometimes of _fearful_ proportions.

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Heads may be made to nod, faces to laugh; eyes may be made to roll, teeth to gnash; crocodiles may be made to swallow tigers; combats may be represented; but one of the most instructive uses of the slides is to make them ill.u.s.trative of astronomy, and to show the rotation of the seasons, the cause of eclipses, the mountains in the moon, spots on the sun, and the various motions of the planetary bodies, and their satellites.

THE PHANTASMAGORIA.

Between the phantasmagoria and the magic lantern there is this difference: in common magic lanterns the figures are painted on transparent gla.s.s, consequently the image on the screen is a circle of light having figures upon it; but in the phantasmagoria all the gla.s.s is made opaque, except the figures, which, being painted in transparent colours, the light shines through them, and no light can come upon the screen except that which pa.s.ses through the figure, as is here represented.

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There is no sheet to receive the picture, but the representation is thrown on a thin screen of silk or muslin placed between _the spectators and the lantern_. The images are made to appear approaching and receding by removing it further from the screen, or bringing it nearer to it.

This is a great advantage over the ordinary arrangements of the magic lantern, and by it the most astonishing effects are often produced.

DISSOLVING VIEWS.

The dissolving views, by which one landscape or scene appears to pa.s.s into the other while the scene is changing, are produced by using two magic lanterns placed side by side, and that can be a little inclined towards each other when necessary, so as to mix together the rays of light proceeding from the lenses of each, which produces that confusion of images, in which one view melts as it were into the other, which gradually becomes clear and distinct; the principle being the gradual extinction of one picture, and the production of another.