THE OPHTHALMOSCOPE. 



159 



The subject is asked to stare ahead of him, in order to relax his 

 accommodation, or, what is more effective, an application of atro- 

 pine is made to the eye. This paralyses the endings of the third 

 nerve in the iris and the ciliary body and prevents accommodation, 

 and reflex contraction of the pupil under the stimulus of the strong 

 light. The rays from the retina, arising at the principal focus if 

 the eye is a normal one, pass out after refraction parallel to one 

 another. If these are to be clearly focussed on the observer's 

 retina his eye must be unaccommodated. He must not look there- 



FIG. 44. To illustrate the use" of the ophthalmoscope. Upper diagram shows the 

 course of the illuminating rays and of the outgoing rays, by the direct method. Lower 

 diagram shows the course of outgoing rays when the indirect method is used; the 

 illuminating rays have been omitted. 



fore at the retina which he wishes to see but must gaze through it 

 into the distance. If the observed eye is shortsighted, the prin- 

 cipal plane, the plane from which rays would have to come to be ren- 

 dered parallel after passing out through lens and cornea, is some- 

 what in front of the plane of the retina. Light rays from the 

 retina, therefore, since they diverge less than would rays from 

 the principal plane, are too much refracted by the eye media and 

 converge after refraction. If they are to make a sharp focus on 

 the unaccommodated eye of the observer they must be rendered 



