Page:Popular Science Monthly Volume 24.djvu/834

814 The fact is, there is no phosphorescence in the eyes of animals—at least, so far as my knowledge extends, none has been demonstrated; and, that it is absent from the eyes of the cat, Professor Tait can demonstrate conclusively for himself, by taking a cat, be it ever so black (and these I believe are supposed to have the luminous power in the greatest degree), into a completely dark room where there can come no ray of extraneous light, and he will find that the eyes can not generate enough light to make even the darkness visible.

The real cause of the luminosity of the eyes of animals in the dark is now thoroughly understood by physiological opticists and by many practical oculists, and depends upon the well-demonstrated laws of the refraction and reflection of light. For a clear apprehension of the phenomenon, however, it is necessary to understand the properties of the eye as an optical instrument.

The office of the eye as an optical instrument, pure and simple, is to bring rays of light to a focus on the membrane at the back part known as the retina, in such a manner that small and inverted images of external objects shall be formed there. For this purpose there is a general plan, which is subject, f however, to more or less variation in different animals. The basis of this plan is the camera-obscura, in which the box is represented by the hollow globe or ball of the eye, the small aperture through which the light enters, by the pupil, and the lens by which the inverted and reduced images of external objects are formed, by the refracting surfaces of the eye, which are usually two—the cornea, or clear part of the front of the eye, and the crystalline lens.

Now, the eye, in its capacity of optical instrument, is obedient to the same laws as any other apparatus reflecting and refracting light. It may astonish some to be told that the eye reflects the light passing into it. It was for a long time believed that all light that entered the eye was in some manner consumed there, and that none ever found its way out again. It was considered one of the functions of the choroid or pigmented coat of the eye to absorb such light as was not used in the formation of the image. The basis of this opinion was that, under ordinary circumstances, no matter how bright the light may be in which the eye is looked at, the pupil always appears black. But no fact is more clearly demonstrated now than that the eye does throw back a large part of the light which enters its pupil.

One of the fundamental principles of optics is what is called the law of conjugate foci. This is readily understood by means of the accompanying diagram (Fig. 1). If the object is at a, the lens I will form an image of that object at c. The law of conjugate foci is that the image can exchange places with the object and the object with the image, and the result be still the same. That is to say, if the object were placed at c, its image would be formed at a. Or, expressing it in another way, the rays of light follow the same lines,