674 



THE SENSES 



positions of the candle and the observer, with reference to the axis of 

 vision of the eye observed. 



If the observer protects his own eye from the direct light of the candle 

 by a blackened cardboard between his eye and the candle, and asks the 

 observed person to accommodate now for near objects, now for far, keeping 

 the axis of vision constant, he will be able to note that the middle image, 

 i,e., the one from the anterior surface of the lens, changes in size and in relative 

 position with reference to the other two, which are essentially constant. With 

 near accommodation this image becomes smaller and seems to move toward 

 the image from the cornea; with far accommodation it becomes larger and 

 appears to move to the image reflected from the posterior surface of the lens. 

 This shows that the act of accommodation consists in a change in the con- 

 vexity of the front of the lens. 



1 6. The Phakoscope of Helmholtz. This classical instrument was 

 invented by Helmholtz to demonstrate the act of accommodation, as out- 



FIG. 483. Disc of Concentric Lines for the Astigmatic Test. 



lined in the second paragraph of the preceding experiment. Repeat the 

 preceding experiment, using this instrument in a dark room. 



17. Astigmatism. Astigmatism is a term used to describe the con- 

 dition of unequal curvature of the refracting surfaces of the eye in the 

 different meridia. The cornea is the surface which usually shows the greatest 

 astigmatism. This defect is demonstrated by numerous forms of astigmatic 

 charts, the most serviceable of which are the barred-letter test type, the clock 

 dial, or the dials shown in figure 463 or 483. Hang an astigmatic dial at a 

 distance of six meters and test the right and left eyes separately, as follows: 

 When the vision is focussed on the center of the dial, if the eye is normal, 

 the three bars in each radius of the clock dial will be seen with equal distinct- 

 ness and have sharp black lines. In an astigmatic eye one or more of these 

 radii will appear sharp and distinct, while the other will appear dim and 



