THE BLIND SPOT IN THE EYE. 125 



contrast with the form which is general, though not 



universal among the lower animals, that I think it 



will not be out of place to 



mention a very simple and 



beautiful experiment by 



which every one can satisfy 



himself that it is so. 



One result is that we have 

 in each eye a blind spot, that 

 at which the nerve enters. 

 Tarn the present page, so 

 that the white circle is in 

 front of the left eye and the 

 small cross in front of the 

 right. Til en close the right 

 eye, look steadily across at 

 the cross with the left, 

 and move the book slowly 

 backwards and forw^ards. 

 At one particular distance, 

 about ten inches, the white 

 circle will come opposite 

 the blind spot and will 

 instantaneously disappear. 

 Across an ordinary room, if 

 a man stands in front of a 

 screen, his head may in the 

 same way be made entirely 

 to vanish. 



The ordinary vertebrate 

 eye consists of two main 

 divisions : the refractive ^'^°- ^°- 



part, which is a modified portion of the skin ; and the 



