Page:Popular Science Monthly Volume 1.djvu/478

464 is replaced by a dim plate of glass. Now cast a glance at Fig. 2; and first suppose the box to be round; next, instead of the wooden wall, an organic tissue; and the window glazed with a transparent organic coat or tunic, instead of with a crystal lens, and which fulfills the same purpose as a collective lens, and strengthened by one or more lenses placed one behind the other. Farther, suppose, instead of the blackened inside of the walls of the box, the organic sclerotic coat overlaid from the inside by a second dark-colored tissue; and, lastly, the retina at the farther end as sensitive plate, and you have certainly an imperfect, but still, as far as an outline goes, a good general idea of the chief parts of the eye.

To the clearer understanding of these parts, the figure is provided with letters.

The tissue marked at the different sections with an S, is the enveloping tissue called the sclerotic or sclerotic coat.

To the front, overlapped by the above, lies the transparent tissue, the cornea, C, which represents the window, and at the same time contributes essentially to collecting the rays of light. At the back enters the optic nerve, which, spreading out to right and left within the sclerotic, receives the name retina, likewise marked with an N.

The chief business of the lens, L, lying well back, and rendered perfect by the humors (K, aqueous, K', vitreous), which fill the spaces or chambers, is the refraction of the rays, whose admission has already been prepared by the cornea.

And, lastly, overlying the interior surface of the sclerotic, is the choroid with its pigment, being the substitute for the black paint in the camera. You find it marked with an A.

Now, if this eye with its cornea, like a camera-obscura with its window, is turned on the objects of the outer world, we shall behold what Fig. 3 shows us: The light proceeding from a point A, beyond the eye, throws a pencil of rays on the cornea; this is already refracted here and there on the surface of the lens, but in a manner so as to collect all its rays again in the one point a of the retina. This a, then, is the image-point of the object-point A. In the same manner, b becomes the image-point of the object-point B, and all the object-points between A and B will find their image-points on the retina between a and b. In a word, an inverted perspective image of all the objects comprehended in the space A B will be found reflected on the retina.

Let us now examine a little more closely the structure of the eye, together with the object it is designed to serve, taking the separate parts in the direction from without inward.

The sclerotic, a stout and not very elastic coat, wants no further description. On the other hand, however, the cornea, as the transparent window, deserves your whole attention. In the construction of the cornea Nature has had to overcome exceptional difficulties. If you remember how apt every organic body exposed to the air is to fall a