THE EYE AS A SENSORY APPARATUS 



233 



Fig. 94, holding the book vertically in front of the face, and mov- 

 ing it to and fro. It will be found that at about 25 centimeters 



FIG. 94. 



(10 inches) off the white circle disappears; but when the page is 

 nearer or farther, it is seen. During the experiment the gaze must 

 be kept fixed on the cross. There is thus in the field of vision a 

 blind spot, and it is easy to show by measurement that it lies where 

 the optic nerve enters. 



When the right eye is fixed on the cross, it is so directed that 

 rays from this fall on the fovea (y, Fig. 95). The rays from the 

 circle then cross the visual axis at the nodal point, 

 n, and meet the retina at o. If the distance of 

 the nodal point of the eye from the paper be /, 

 and from the retina (which is 15 mm.) be F, then 

 the distance, on the paper, of the cross from the 

 circle will be to the distance of y from o as / is to F. 

 Measurements made in this way show that the 

 circle disappears when its image is thrown on the 

 entry of the optic nerve, which lies to the nasal 

 side of the fovea. 



2. The above experiment having shown that 

 light does not act directly on the optic nerve- 

 fibers any more than it does on any other nerve- 

 fibers, we have next to see in what part of the 

 retina those changes do first occur which form 

 the link between light and nervous impulses. 

 They occur in the outer part of the retina, in the rods and cones. 

 This is proved by what is called Purkinje's experiment. Take 

 a candle into a dark room and look at a surface not covered 

 with any special pattern, say a whitewashed wall or a plain 



