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 7600, measured in Ang- 

 strom units (1 Au = 10,000,000 mm.), and, on the other, the ex- 

 treme violet, having a wave length of about 3900. 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. 141. Demonstration of blind spot, left eye (Stifel's figure): Hold the black circle in 

 front of the left eye at a distance of about 10 inches. Close the right eye. Place a pencil 

 point at A and draw it slowly along the line ABC, keeping the left eye on the pencil point. 

 At a certain distance beyond A, for example, at B, the white spot in the black circle will dis- 

 appear (falls on the blind spot), but will reappear when the pencil point is moved further out- 

 ward, at C for example. By marking the points at which the white spot disappears and re- 

 appears the diameter of the spot is given for that meridian and distance of projection. In 

 the same way the diameter may be marked for the other meridians and the form of the blind 

 spot be obtained. 



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 



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