chap, xxviii.] THE OPHTHALMOSCOPE. 353 



near enough, an image is seen of the retina, optic 

 nerve entrance, etc. The image is virtual, erect, and 

 magnified, as represented in Fig. 159, ed. This 

 image can be obtained only if the observing and the 

 observed eyes are both focussed for an infinite 

 distance. This is practically secured by making the 

 person look to a distant point, say at the other end of 

 the room, and by the observer looking as if to a 

 distance. This condition, however, it is often not 

 possible to obtain. The result is that the reflected 

 rays from the eye are not accurately focussed on the 



Fig. 159. The Ophthalmoscope with Erect Image. 



retina of the observing eye, and circles of diffusion 

 are formed. 



Under such circumstances the use of a diverging 

 lens will render the image distinct. The action of 

 such a lens is shown in Fig. 159. 



The observed eye is A, and the small arrow a 

 represents a part of the retina 011 which light is 

 thrown by the concave mirror. Rays from a passing 

 outwards would be converged by the media of the eye, 

 and would come to a focus at 6. An image would 

 thus be formed, 6c, magnified, and inverted, a real 

 image moreover. But by the action of the concave 

 lens B (whose focal distance is pB) the rays are made 

 to diverge, and thus a virtual image is formed behind 

 the eye, an image larger than the object, but erect. 

 That is, the rays from a, which reach the observer's 

 eye, appear by the action of the lens to proceed from 

 the point d. . 

 x 8 



