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 as usually given are, on 

 the one hand, the extreme red rays with a wave length of 7600, 

 measured in Angstrom units (1 ku = lo.ooo.ooo mm.), and, on the 

 other, the extreme violet, having a wave length of about 3900. 

 If the eye is kept in the dark for some time (dark-adapted) its 

 sensitivity to light is increased and the limits of the visible spec- 

 trum are said to be extended to take in the range of vibrations 

 from 9000 to 3300. The part of the retina stimulated by these 



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. 



vibrations is supposed to be the layer of rods and cones. To reach 

 these structures the light must pass through the other layers of 

 the retina. That the rods and cones are the structures that react 

 to the hght stimulation is indicated by their structure and their 

 connections and by such facts as the following: 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 externally to these 

 vessels. In the fovea centrahs, 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 



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