THE SENSES 921 



becomes dark from one side as if covered by a shadow. This 

 shadow will move in the same direction in which the mirror is 

 rotated or in the opposite direction, according to whether the 

 observer is farther from the observed eye than its far- point, or 

 between the eye and the far-point. If the observer is exactly 

 at the far- point, no direction of movement of the shadow can be 

 made out, but the pupil in its whole extent is either illuminated 

 or altogether dark. In this way the distance of the far-point of 

 a myopic eye can be easily determined by a metre rule, and 

 from this the degree of myopia. If the far-point is either too 

 near, as in strong myopia, or too distant, as in weak myopia 



FIG. 400. INDIRECT METHOD OF USING THE OPHTHALMOSCOPE. 



The rays of light issuing from E', the observed eye, are focussed by the biconvex 

 lens L, and a real inverted image of a portion of the retina of E', magnified four 

 or five times, is formed in the air between the lens and the observing eye E. This 

 image is viewed by E at the ordinary distance of distinct vision (10 to 12 inches). 

 (The exaggeration of the size of the mirror makes it appear as if some of the rays 

 from the lamp passed through the lens before being reflected from the mirror. 

 This would not be the case in an actual observation.) 



and emmetropia, or behind the observed eye, as in hypermetropia, 

 it can be brought to a convenient distance by interposing suit- 

 able lenses. The observer then determines the far-point exactly 

 by moving his eye nearer to or farther from the observed eye, 

 or, keeping his own eye fixed, by bringing the far-point of the 

 observed eye to coincide with it by inserting lenses (Practical 

 Exercises, pp. 994, 995). 



The phenomenon depends upon the interruption which the light 

 proceeding from the observed retina experiences first at the margin 

 of the pupil of the observed eye, and then at the margin of the hole 



