CHAPTER XVIII. 



THE PROPERTIES OF THE RETINA VISUAL STIMULI 

 AND VISUAL SENSATIONS. 



The Portion of the Retina Stimulated by Light. The normal 

 stimulus to the sensory cells in the retina is found in the vibrations 

 of the ether, the waves of light. When sunlight is passed through a 

 prism the waves of different lengths are dispersed, and those capable 

 of stimulating the retina form the visible spectrum extending from 

 red to violet. The limits of the spectrum are, on the one hand, the 

 extreme red rays with a wave length of T winnr mm - an d vibrating 

 at the rate of about 390,000,000,000,000 a second, and, on the other, 

 the extreme violet, having a wave length of about i oo Sfinnr mm - an d 

 a rate of vibration of 757,000,000,000,000 a second. The part of 

 the retina stimulated by these vibrations is supposed to be the layer 

 Of rods and cones. To reach these structures the light must pass 



Fig. 134. To demonstrate the blind spot. Fix the center of the cross with the right 

 eye, then move the book slowly to or from the face. At a certain distance the image of 

 the large circle to the right will disappear. At this distance the image of the circle falls 

 on the optic disc. 



through the other layers of the retina. That the rods and cones are 

 the structures that react to the light stimulation is indicated by 

 their structure and their connections and by such facts as the follow- 

 ing: Under certain conditions, which are described below, the 

 shadows of the retinal vessels and the contained corpuscles may be 

 seen, a fact which indicates that the perceiving structures lie ex- 

 ternally to these vessels. In the fovea centralis, in which vision is 

 most perfect, the layers of the retina are thinned out until practically 

 only the rods and cones remain to be acted upon. That the optic 

 nerve fibers themselves are not acted upon by light waves is proved 

 by the existence of the blind spot. The termination of the optic 

 nerve within the eyeball, the optic disc, lies about 15 degrees to the 

 nasal side of the fovea and has a diameter of about 1.5 mms. From 



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