688 



THE POPULAR SCIENCE MONTHLY. 



the same as the path of the incident rays; for if, at the point where 

 an image of an object has been formed by a lens, we place an exactly 

 similar object instead of the image, then an image will be formed in 

 the exact position of the first object, and of equal size. We see from 

 this experiment, therefore, that the rays of light, which are emitted 

 by an image formed upon the retina, must return to the object from 

 which they originally proceeded. 



If, therefore, a light is placed before any eye which we wish to 

 examine, the rays will all be reflected by the eye into the light, aud 

 we are unable to intercept them by our own eye, because we should 

 hide the light by placing ourselves between it and the eye under ex- 

 amination. By means, however, of a transparent plate of glass, this 

 obstacle may be overcome, and the eye examined when illumined, in 

 the manner represented in Fig. 3. C is the eye under observation, B 



A<A 





Fig. 3. 



the observer's eye, and the plate of glass, S, forms an angle of 45 

 with the line between the two eyes. The rays emitted by the lighted 

 candle, A, strike the glass plate, S, and are partly reflected into the 

 eye, which they illuminate. The rays reflected by the eye, C, again 

 strike the glass plate, which some of them penetrate, and pass into 

 the eye of the observer, and the remainder return to the light, A. 

 The pupil of the eye, C, may now be seen brightly illuminated, and 

 even the illuminated retina can be seen more or less distinctly. The 

 rays emitted by the image formed upon the retina, which pass through 

 the glass plate, would form an image at a, which is at the same dis- 

 tance from the glass plate as A. The rays are, however, intercepted 

 by the observer, H, who is thus enabled to examine a part of the 

 retina. 



In fact, a piece of window-glass placed in an oblique position, as 

 described above, is the simplest form of an ophthalmoscope, and may 

 easily be arranged by any one who wishes to make the experiment for 

 himself. An ordinary piece of glass is sufficient for the purpose, if 

 placed in the same position, relatively to the eye under observation 

 and the light, as that shown in the figure. It is well to place a 



