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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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Structure of the Eye.

375 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

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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. In 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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image is formed. Thence the optic nerve, carrying to the brain the impression produced by the vibrations of the ether on the nervous system of the retina, enables us to perceive external objects. In accordance with this explanation, we should see objects reversed, and not in their natural position. The inversion of images in the eye has greatly occupied both physicists and physiologists, and many theories have been proposed to explain how it is that we do not see inverted images of objects. Some have supposed that it is by custom, and by a regular education of the eye, that we see objects in their true position, that is to say, in their position relative to us. The visual impression becomes corrected by the impression of other senses, such as that of touch. Müller, Volkmann, and others contended that, as we see everything inverted, and not simply one object among others, nothing can appear inverted, because terms of comparison are wanting. It must, however, be admitted that none of these theories is quite satisfactory.

371. Distance of distinct vision. Short and long sight.----We know that in double convex lenses the distance of images from the lens increases or diminishes as the object is approached or receded (325). Hence, according to the distance of the objects looked at, it would seem that the image formed by the crystalline should be sometimes formed exactly on the retina, and sometimes a little in front of or behind this membrane. Only objects placed at a certain distance should then be seen distinctly; all those nearer or further should appear confused. This does not occur with a wellshaped eye, for it sees objects distinctly at very different distances; whence it is concluded, that the eye has the power of rapidly accommodating itself, so that the image is always formed exactly upon the retina.

Yet, though the eye can well distinguish objects at very different distances, there is in the case of each person a distance at which objects are more distinctly seen than at any other. This distance is called the distance of distinct vision; it varies with different persons, and often in different eyes in the same individual; for small objects like print it is usually about ten to twelve inches.

Those who can only see well at shorter distances have a defect in the shape of the eye; they are said to be myoptic or short-sighted, from two Greek words which signify close the eyes; for myoptic persons, in order to see more distinctly, do in fact involuntarily half close the eyes. If the distance of distinct vision is greater than ten or twelve inches, that is also due to a malformation of the eye, and those affected by it are called long-sighted or presbyoptic, from a Greek word which signifies aged, for this defect is usually met with in aged persons.

Jyopy, or short-sight, results from too great a convexity of the cornea, or of the crystalline. The eye too convergent, the rays of light are refracted in such a manner, that instead of forming their focus exactly on the retina, they form it a little in front, and therefore the image which this membrane perceives is confused. But if objects are approached to the eye, the image recedes, and is formed exactly on the retina, when the objects are sufficiently near, which explains why short-sighted persons only see objects when they are very close. They can also see more distinctly by contracting the pupils, or by looking through a small hole perforated in a card; for then, as the diameter of the luminous pencil which penetrates into the eye, the rays mainly penetrate the crystalline at the centre, and being therefore less affected by its excess of convexity, they form the focus at a greater distance. Myopy is mainly met with in young people; as they grow older, the convexity of the eye diminishes, so that their sight generally becomes better, when that of other people becomes worse.

Presbytism, or long-sight, is due to the flattening of the crystalline, as the eye is then no longer sufficiently converged, the rays instead of forming their focus on the retina tend to form beyond it, whence it arises that the eye only observes a confused object. But as the objects recede, the image comes nearer the crystalline, and is ultimately formed exactly on the crystalline, when objects are sufficiently distant ; which explains why long-sighted persons only see objects when they are distant.

Short-sight is remedied by the use of diverging lenses before the eyes ; by the action of these glasses, as the pencil is spread out before entering the eye, the focus of the crystalline is receded as far as the retina, provided the degree of divergence of the glasses is suitably adapted to the excess of convexity of the crystalline. For far-sight, on the contrary, condensing lenses should be used, so as to correct the want of convexity of the crystalline. As the rays then become more convergent before entering the eye, the image, which would otherwise be formed beyond the retina, approaches it, and is ultimately formed exactly upon it.

For a long time double concave lenses were exclusively used for short-sight, and double convex for far-sight. We strongly recom

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