8 14 THE POPULAR SCIENCE MONTHLY. 



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 dem- 

 onstrate 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, however, to more or less variation 

 in different animals. ITie basis of this plan is the camera-ohscura, 

 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, 



