Page:Popular Science Monthly Volume 24.djvu/836

816 of an eye which has been illuminated. But, if we were able by any contrivance to place our eye in the position of the source of light, we would be able to catch the rays coming from the bottom of the observed eye, and it would appear illuminated. Now, there is such a contrivance, and it is called the ophthalmoscope, and it owes its existence to the genius of Professor Helmholtz. The principle of its construction is so simple that the wonder is that no one ever thought of it before; but never, until the year 1851, had any one ever seen in anything like detail the interior of a living eye. If you take a piece of bright tin and punch a small hole in it, and, placing the hole directly in front of your own pupil, throw the light from a lamp into the eye of a child, the pupil, instead of appearing black as it usually does, will be of a beautiful yellowish-red color. This is because you have, to all intents and purposes, put your eye in the place of the source of light. For the light reflected from the surface of the tin is that which passes into the eye, and it must come back to it after reflection. The eye placed behind the hole catches the small quantity which would fall on that part, and therefore sees the surface from which it comes, illuminated. This is the principle of illumination of the bottom of the eye, and, when you have your object sufficiently well lighted, it is only a matter of optical appliance to see it distinctly and in great detail. This digression is designed to show that, when we have favoring circumstances, by the action of well-known optical laws, the eyes of animals appear illuminated, and that it is not necessary to call in the supposition of phosphorescence to account for the phenomenon.

But, in the case of some animals, the eyes appear to shine without the intervention of any optical means, however simple. This, however, is only apparent, for the principle of illumination is applicable here as in the other cases.

In the case we have supposed, the retina, which in this instance is the reflecting surface as well as the membrane on which the image is formed, was found at the focus of the refracting surfaces of the eye. But this condition is met with only in what is accepted as the perfect optical state of the eye. As can be readily understood, the retina may lie either in front or behind the focus of the refracting media—that is, the eye may be too long or too short for its focus, and unfortunately such conditions are but too common. When an eye is too long, it is said to be near-sighted or myopic; when too short, it is far-sighted or hypermetropic.

The change in the position of the retina, then, must exercise an influence on the direction of the rays that are reflected from it. From the well-demonstrated properties of lenses we know that, when rays of light coming from a point at the focus of a lens pass through it, they are rendered parallel; when they come from a point within the focus, they are spread out, or rendered divergent; and when from beyond