816 THE POPULAR SCIENCE MONTHLY. 



torn 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 be- 

 cause 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 illumina- 

 tion of the bottom of the eye, and, when you have your object suffi- 

 ciently 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, how- 

 ever, 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 per- 

 fect 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 unfor- 

 tunately 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 



