SECTION VIII 

 VISUAL SENSATIONS 



THE retinal changes which we have just described as occurring in the retina 

 on exposure to light give us very little information as to the nature and 

 conditions of the physiological activity excited in this organ by the physical 

 stimulus of light. We are therefore driven to use as our criterion of these 

 physiological processes the changes excited in consciousness, and the greater 

 part of our knowledge of the physiology of vision is derived from examination 

 of such of our own sensations as have their primary origin in the retina. 

 How far these sensations can be regarded as having their seat in the retina, 

 how far they are determined by physiological changes in the visual and ad- 

 jacent portions of the brain, it is not possible to say. We are only able to 

 deal with the sensations as they spring ready formed into our consciousness. 



NATURE OF THE STIMULUS 



The word * light,' as employed by physicists, implies a particular kind 

 of energy, which, arriving at the retina in a certain way, excites in us a 

 sensation of light. The conception is therefore primarily physiological. 

 Every material substance is endowed with a certain amount of internal 

 energy, the index to which is its temperature. In virtue of this energy 

 it is constantly radiating energy at a greater or less rate through the sur- 

 rounding ether, its internal energy at any given moment being determined 

 by the balance between the amount of energy it gives off and the amount 

 of energy which it receives from surrounding bodies. This radiant energy 

 is transmitted through space as transverse oscillations of the ether at the 

 rate of about two hundred thousand miles per second and with very variable 

 wave-length and rate of oscillation. The whole energy available to us on the 

 surface of the earth is derived from that portion of the radiant energy of the 

 sun which is intercepted by the earth. 



Since the velocities of transmission of rays of various wave-lengths differ 

 as these rays pass through a dense medium, such as glass, it is possible to 

 break up the compound waves of radiant energy arriving at us from the sun, 

 or emitted by any hot body, by allowing them to pass through a prism. 

 When the luminous solar rays are passed in this way through a prism we get, 

 as is known, a spectrum, the rays which are refracted the least being red, 

 while those which are most refracted are violet. Between these two 

 extremes we have rays of the following colours orange, yellow, green, blue, 

 indigo, which merge one into the other without any perceptible break. The 



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