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 adjacent 

 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 surrounding 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 



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