762 THE SENSES 



15. Purkinje-Sanson's Images. Examine the eye of another person 

 in a dark room as follows: With the observing eye focus for a far object, 

 let the observer hold a candle slightly to one side of the axis of vision and 

 about one foot from the eye. If the observer looks into the other eye from 

 the side opposite the candle, he will be able to see three reflected images, 

 figures 457 an d 45 8. One, from the anterior surface of the cornea, is 

 bright and distinct, and of medium size and erect. In the middle of the 

 pupil there will be a second image, larger and quite dim. This is a reflec- 

 tion from the front of the lens. The third image, reflected from the posterior 

 surface of the lens, will seem to be farther back in the eye, quite small and 

 inverted. These images can all three be seen at once with careful adjust- 

 ment of the relative positions of the candle and the observer, with refer- 

 ence to the axis of vision of the eye observed. 



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

 fay 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 con- 

 stant. 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 convexity of the front of the lens. 



16. The Phakoscope of Helmholtz. This classical instrument was 

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

 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. The 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 focused 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 

 indistinct, the relative difference depending upon the degree of astigmatism. 

 Note the meridian of astigmatism in the right and left eyes separately. Use 



