THE BLIND SPOT. 583 







epithelial protoplasm, which is the really effective part of the 

 retina. Of this we have the following evidence : 



1. The fact that the rods and cones must be regarded as the 

 real anatomical nerve terminals of the optic nerve. 



2. Where the optic nerve enters the eyeball and the nerve 

 fibres are fully exposed to the light, there are no rods and cones. 

 This part, the optic disk, cannot appreciate light, and hence is 

 called the " blind spot." This fact shows that the nerve fibres 

 are quite insensitive to light, and that we must look to the ter- 

 minals for its appreciation. The existence of the blind spot can 

 be demonstrated as follows : Shut the left eye, and hold the left 

 thumb, at ordinary reading distance, in front of the right eye. 

 While the eye is fixed on the left thumb, bring the right thumb 

 to within about four inches of it, and move it slowly an inch or 

 so from side to side. A little practice soon enables one to find a 



a o AA x 



A 



place when the right thumb nail disappears. The blindness of 

 this part of the retina also can be demonstrated by steadily fixing 

 the right eye the left being closed on the small letter " a " 

 and moving the page to or from the eye very slowly ; a distance 

 may thus be reached when the large letter "A" is quite lost. On 

 approaching the page the eye still fixed on "a" when "A" 

 has become invisible, it reappears from the inner side, " x" first 

 coming to view ; on withdrawing the page it comes into view from 

 the outer side, " o" being first seen. This blind spot is not noticed 

 in ordinary vision, as we have habitually overcome the deficiency 

 since infancy by our judgments being derived from two eyes. 

 By rapid movements one eye hides the deficiency, as seen when 

 attempting the experiment just described. 



3. The fact that when the eye is illuminated in a peculiar way 

 we can see the shadow of the blood vessels which lie in the inner 



