REFRACTION IN THE EYE 475 



Depress the paper diaphragm until the margin 

 of the pupil lies against the mirror. The black 

 line will disappear, because the ray J E, reflected 

 from the back of the diaphragm, is intercepted. 

 The space P H is closed (Helm hoi tz). 



2. In a dark room repeat Experiment 1 upon 

 the eye. The iris will be the diaphragm, 1 1', and 

 the anterior surface of the crystalline lens will 

 be the convex mirror. The light and the ob- 

 server's eye should be placed as in Fig. 66. 



The bright image of the light formed by the 

 cornea, should be neglected. Near this image 

 are two others, very much fainter. The larger 

 of the two is indistinct and upright ; it is re- 

 flected from: the anterior surface of the crystal- 

 line lens. The smaller is a sharp, inverted image 

 from the posterior surface of the lens. By mov- 

 ing the glass lens the light may be thrown at 

 will on all 'parts of the border of the pupil. 



No dark line or image of the posterior surface 

 of the iris will be seen. The margin of the iris 

 lies upon the lens. 



Changes in the Lens. 1. Direct the subject to 

 cover one eye. Place a needle at the near point 

 of the other eye in line with some distant object 

 that can be clearly seen. The two objects must 

 be kept accurately in line throughout the experi- 

 ment. Let the observer stand at one side of and 



