VISION 



i35 



method of skiascopy (shadow test). It depends upon the following 

 observation: When one throws light from a little distance with a 

 concave mirror into an observed eye and then rotates the mirror 

 slowly around the long axis of the handle, one sees that the pupil, 

 which at first was completely illuminated, 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 



Fig. 435. 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.) 



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 and emmetropia, or behind the observed eye, as in hyper- 

 metropia, it can be brought to a convenient distance by interposing 

 suitable 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. 1109, mo). 



