THE EYE AND THE SENSE OF SIGHT 



123 



object without moving the eyeball; the pupil and iris next the 

 observer will be seen to move forward, owing to the increased curva- 

 ture of the anterior surface of the lens. 



53. Movements of the Iris. In the preceding experiment the diame- 

 ter of the pupil was smaller when the eye was accommodated for near 

 objects, but dilated on changing to distant vision. Close one eye, 

 and by aid of a mirror observe the size of the pupil of the other eye. 

 Then open the closed eye ; the pupil of the othar eye contracts. Cover, 

 with the hands, another person's eyes. On suddenly removing the 

 hands, the pupils are seen to contract. 



54. Astigmatism. Close one eye and look at the radiating lines in 

 Fig. 72. Notice which lines, if any, appear with the greatest black- 

 ness and distinctness. Try the other eye ; do the two eyes agree ? Look 

 at the concentric circles of Fig. 72. In what portion of the figure, if 



Fig. 72. 



any, do the lines appear clearest ? The teacher should obtain, from 

 an optician, one of the charts commonly used in preliminary tests for 

 astigmatism. With this he can detect some of the more pronounced 

 cases of this optical defect among his pupils. It may happen, how- 

 ever, that very great defects cause little disturbance, since if the two 

 eyes are astigmatic on different axes, one eye may correct the defect 

 of the other. 



55. Near sight and Farsight. These are common defects, and 

 their causes should be illustrated by use of the apparatus described 

 in Ex. 50. If the teacher will perform some of the simpler tests 

 for optical defects of the eye (and almost any local optician will loan 

 the necessary apparatus, and give instructions), he may not only inter- 



