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chloride of silver. The picture is afterwards immersed in a bath of chloride of gold, which gives it tone.

367. Positives on glass.-Very beautiful positives are obtained by preparing the plates as in the preceding cases; the exposure in the camera, however, is not nearly so long as for the negatives.

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The picture is then developed by pouring over it a solution of protosulphate of iron, which produces a negative image; and by

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afterwards pouring a solution of cyanide of potassium over the plate, this negative is rapidly converted into a positive. It is then washed and dried, and a coating of varnish poured over the picture. 368. Diorama.-The name diorama is derived from two Greek words which signify viewed through, and is applied to pictures painted on muslin or on calico, so as to represent two opposite effects like the polyorama, according as the pictures are seen by reflection or by transmission. ·

The picture is arranged vertically in a dark 100m as represented in fig. 289. The first effect, that painted on the front of the cloth, is illuminated by reflection: the second, that painted behind, is illuminated by transmission. With this view light enters through a window, M, in an upper story, and is sent to a screen, which reflects it towards the picture, and lights it from the front : behind the picture is another window, N, which, when open, lights it in front. The shutters, NN, being closed, the spectators first see the subject on the front of the cloth. By a simple arrangement, a shutter, A, which slides without noise in two grooves, is made to advance, and when the picture is scarcely illuminated, by degrees the shutters, NN, are opened; and then the picture painted on the other side of the cloth appears through it, and is substituted for the former one.

The diorama was invented by Daguerre, who had great skill in this kind of painting. The above figure represents the valley of Goldau before the terrible landslip, which took place on September 2, 1806. At the moment light was intercepted by the screen, lightning flashed, thunders groaned, and there were all the effects of a violent storm. On the return of day, the rocks had given way, the lake had been partly filled up, and the chalet destroyed; in short, the image of ruin and desolation was reproduced with astounding fidelity.

369. Ghost scenes.—We will give here a description of a curious optical effect, which was first introduced some years ago in the London theatres, under the name of ghost scenes.

In order the more readily to understand the appearance of these spectres, let us recal an effect which everyone has observed. When at night on a railway we look at the windows of carriage doors, we see a pale and indistinct image of the travellers inside. This is an effect of reflection from the panes, which reflect the light that illuminates persons and objects placed in the compartment; and the faint light of the images arises from the fact, that the panes allowing great part of the light to be transmitted, send very little towards the

observer. A similar effect is produced when in the evening, in a well-lighted street, a window front, which is little or not at all

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lighted, is looked at. The observer sees his own image and that of the passengers on the other side of the panes. These effects are not seen in full daylight, for the images which tend to be reproduced are effaced by the brightness of the light.

These effects have been utilised in public to simulate the appearance of ghosts. Fig. 290 represents the arrangement of the apparatus intended for this use. On the floor of the stage, and not visible by the spectators, is an actor covered by a sheet, and intended to represent the ghost. Between the actor and the public is a dark lantern, in which is the lime light, which gives an extremely bright light. An assistant directs the light upon the actor, and the white cloth, thus powerfully illuminated, sends its rays towards an inclined sheet of glass, placed near the assistant. This glass, which is silvered, sends almost all the reflected light towards a second sheet, which is not silvered on the same scene. This latter plate acts like those in carriages and in shopwindows, which we have mentioned above, and being traversed by the greater part of the incident rays, send but little light towards the spectator. Yet, as during this time, care is taken that the illumination in the room is very faint, the light is sufficient to give a cloudy image of the actor placed under the stage.

If another actor enters the scene the public see very distinctly through the glass, which is carefully concealed by hangings and decorations; and if this actor is behind the plate at the same distance as the image, he can join his action with that of the ghost, and produce a complete illusion.

The same effects are produced with a single plate, but as its obliquity tends to give inclined images, to rectify them, the actor under the theatre must hold himself so much inclined as to render his play very difficult. With the two sheets represented in the above figure, the actor retains his natural position.

VISION AND STEREOSCOPE.

370. Structure of the eye and mechanism of vision.-Although the description of the eye belongs to physiology rather than to physics, we may give an account of this organ, which is not merely a true optical instrument, but an instrument of inimitable perfection; for it has neither spherical nor chromatic aberration; and has moreover the remarkable property of adapting itself at once to see distinctly at all distances, which the best optical instruments do not do.

The eye is almost spherical in shape, and is surrounded by several membranes, which fig. 291 represents open from back to front. The front part of the eye is a perfectly transparent membrane, c, called the transparent cornea, and which is commonly called the white of the eye. At a small distance behind the cornea is a membranous diaphragm, hi, called the iris; it constitutes the variously coloured disc which appears in the middle of the white of the eye, and to which is due the colour. In the centre of the iris is an aperture called the pupil; in man this is circular, and in the cat narrow and elongated, and through it rays pass into the eye. Behind the iris, but very near it, is the crystalline, which is a transparent mass, having the shape and fulfilling the functions of a double convex lens. The whole of the back part, from the crystal

Fig, 291.

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line to the bottom of the eye, is filled with a gelatinous transparent mass, like white of egg, which is called the vitreous humour. front of the eye, between the crystalline and the cornea, is a perfectly transparent liquid called the aqueous humour. The whole of the back inside part of the eye is lined with a soft, whitish, transparent membrane, R, called the retina; it is nothing more than the extension of a nerve, N, which proceeds to the brain, and transmits the sensation of vision, whence it receives the name optic nerve. Behind the retina is a second membrane, C, called the choroid, which is impregnated by a black matter, that absorbs all rays which should not coincide in vision. Lastly, a membrane, S, the sclerotica, surrounds the whole eyeball behind, and joins the transparent cornea in front.

These details being known, we may easily account for the mechanism of vision; for the eye is nothing more than a small camera obscura (363), of which the pupil is the aperture, the crystalline is the condensing lens, and the retina is the screen on which the

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