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The perfect transparency of the lens is obviously necessary for distinct vision; cataract, a common cause of blindness, arises from its opacity.

The lens, with its continuing curtain, completely divides the interior of the ball into two compartments, an anterior and a posterior.

(b.) The anterior chamber is filled with a clear, aqueous liquor, called the aqueous humor (Figs. 4 and 5), a small portion of which is behind the iris, but by far the larger portion between the iris and the cornea. The two parts are in connection through the pupil. If the cornea be punctured, the aqueous humor runs out, the clear protuberant part of the eye collapses, and the sight is for the time ruined. If, however, the wound heals without scar, or if the scar be to one side of the direct line of sight, the cornea will fill again and the sight may be recovered.

(c.) The posterior and much larger chamber is filled with a transparent, glassy substance, about the consistence of soft jelly, called the vitreous humor. This humor is in direct contact with the lens and curtain in front, and with the retina over its whole globular surface.

SECTION II.-FORMATION OF THE IMAGE.

The eyeball, as thus described, may be regarded as consisting essentially of two distinct portions, viz.: 1. A nervous expansion, the retina, specialized for responding to light-vibrations; 2. An optical instrument, the lens apparatus, placed in front of the retina, and specially arranged to make the impression of light strong and definite, by means of an image. These two are

entirely different in their origin. In embryonic development, the one is an outgrowth from the brain, the other an ingrowth from the epidermis and cutaneous tissues. These afterward meet and unite to form this

wonderful organ.

Now the sole object of this complex instrument is the formation of a perfect image on the retina. Without images we would perceive light, but not objects; and distinctness of objects is exactly proportioned to distinctness of retinal images. If the image of an object is distinct, the object will be distinct; if the image is blurred, the object, both in outline and in details of surface, will be blurred. If there is no image, no object will be visible. Therefore the image must be a facsimile of the real object, for the apparent object will be a fac-simile of the image.

Conditions of a Perfect Image.-A serviceable image must be sufficiently bright, and perfectly sharp and distinct in outline. Brightness only requires a sufficient amount of light. In order to be perfectly distinct, it is necessary that rays from different points in the object, even the most contiguous, should not mix on the image, but all the rays from each point on the object must be carried to its own point on the image. Now, it is impossible that both of these conditions should be fulfilled, except by some such arrangement as we find in the

eye.

For see suppose the light to enter by a hole only, like the pupil; and, further, in order that there be light enough, let the hole be somewhat large; then the light, diverging from any point, b, Fig. 6, A, of the object a bc, and entering the hole h of diaphragm d d, will form a diverging pencil, and spread out over the whole circle b', on the screen 8 s. Similarly, the rays from a will

spread out and form the circle a', and from c the circle d. Thus it is seen that rays from widely different points in the object mix with each other on the receiving screen; much more, then, would rays from contigu ous points of the object mix. In such a case, the mixing is so great that no recognizable image is formed at all.

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As the hole becomes smaller, the circles of dispersion, a' b' c', become smaller in the same proportion; and, therefore, the light from different points of the object is more and more separated on the receiving screen, and the image becomes first recognizable, then more and more distinct. But, in the mean time, the quantity of light is becoming less and less, and therefore the image fainter and fainter. If we suppose the hole to become a mathematical point, then one ray only passes from each point to the object, and goes to its own place in the image (Fig. 6, B), and the conditions of distinctness are fulfilled; but the image is now infinitely faint, and therefore invisible. If, now, we try to increase the brightness by increasing the size of the hole, in propor

tion as we get brightness do we lose distinctness. We can not get both at the same time.

Experiment.-Let a room with solid shutters be darkened; let one shutter have a hole of a few inches in diameter; cover the hole with an opaque plate of sheet iron, in which there is a very small hole, one tenth to one twentieth of an inch in diameter. If, now, a sheet of white paper be held a little way from the small hole, an inverted image of the external landscape will be seen on the sheet. If we increase the size of the hole, the image will be brighter, but also more blurred.

Illustrations.-Many simple experiments may be made illustrating this principle. A pinhole in a card will make an inverted image of a candle flame. When the sun is in eclipse, it may be examined without smoked glass, by simply allowing it to shine through a pinhole in a card upon a suitable screen. In the shade of a very thick tree-top the sun-flecks are circular like the sun; but during an eclipse they are crescentic, or even annular, according to the degree of obscuration. They are always images of the sun.

Property of a Lens.-Now a lens has the remarkable property of accomplishing both these apparently oppo

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site ends, viz., brightness and distinctness at the same time. If an object, a c, be placed before a lens, L (Fig. 7), then all the rays diverging from any point, b, are

bent so as to come together again at the point b'. Of the divergent pencil, b L L, the central ray passes straight through without deviation; rays a little way from the central are bent a little; rays farther away are bent more and more according to their angle of divergence, so that they all meet at the same point, b'. Similarly all the rays proceeding from a, and falling on the lens, are brought to the same point, a', and from c to the point c', and so also for every intermediate point. Thus an image is formed which is both bright and very distinct if the receiving screen is suitably placed, i. e., at the exact place where the rays meet. The billions of rays from millions of points of the surface of the object are, as it were, sifted out by the law of refraction, and each safely conveyed to its own point in the image; so that, for every radiant point of the object, there is a corresponding focal point in the image. But it is evident that the screen must be suitably placed, for, if it be placed too near, at S' S', the rays have not yet come together; if too far, at S" S", the rays have already met, crossed, and again diverged. In both cases the image will be blurred.

A

FIG. 8.

B

DIAGRAM ILLUSTRATING THE FORMATION OF AN IMAGE ON THE RETINA.

In all dioptric instruments images are formed in this way. It is in this way that images are formed in the eye. In Fig. 8 it is seen that the diverging pencils,

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