CHAPTER XIX. 



EXAMINATION OF THE REFRACTION OF THE EYE AND 

 OF THE INTERIOR OF THE EYEBALL. 



THE OPHTHALMOSCOPE. 



Most of the light which enters the eye passes through the 

 retina and is absorbed by the black pigment layer behind it but a 

 small proportion of the rays are reflected from the retina itself. 

 The rays reflected from each point are refracted again by lens and 

 cornea being bent away from the perpendicular in this case and they 

 pass out in the same general path as was taken by the incoming rays 

 which illuminated the point. That is, they travel back towards the 

 source of light. Since the eye of the observer is not under ordinary 

 circumstances placed at such a source it does not receive any of this 

 light and therefore any pupil at which one looks appears to be black. 

 If however one arranges to have the observer's eye at the source of 

 light, or a very little distance from it, some of the rays reflected from 

 the observed retina can be seen. 



This is the principle on which the ophthalmoscope is con- 

 structed. It not only gives a view of the interior of the eye but 

 also, as we shall see, affords a method of examining its refraction. 

 The instrument is a slightly concave mirror with a tiny hole in the 

 centre of it. One holds it in front of the eye which one wishes to 

 examine and a light is placed beside the eye and a little behind it 

 (L, Fig. 44 A.). The mirror sends rays from this into the eye. 

 LAA' and LBB' in Fig. 44 are two such rays. There will of 

 course be many others, lighting up the whole area between A' and 

 B' and some of them falling on either side. Each point of this 

 illuminated area, reflecting part of the light which it receives, acts 

 as a source of rays, as -is shown for the point X. The observer 

 looks from behind through the little hole and some of the reflected 

 rays (as XY and XZ) returning towards the mirror pass through 

 the hole and afford him a view of the interior of the subject's eye. 



158 



