104 HUMAN PHYSIOLOGY 



This is associated with the perceptions of the three parts 

 differently illuminated. From the previous training of the 

 nervous system we are taught to interpret this as due to a 

 corner. But this interpretation is simply a judgment based 

 upon the sensations, and it may or may not be right. Thus, 

 instead of actually looking at a corner we may be looking at 

 the picture of one. 



From the very first it must be remembered that the modi- 

 fication of our consciousness which we call vision is not 

 directly due to external conditions, but is due to changes set 

 up in our eye by these external states. We do not perceive 

 the object we are looking at, but simply the changes in our 

 brain produced by changes in the eye set up by rays of light 

 coming from the object. 



Usually such changes are set up by a certain range of 

 vibrations of the ether, but they may be set up in other 

 ways e.g. by the mechanical stimulation of a blow on the 

 eye ; but, however set up, they give rise to the same kind of 

 changes in consciousness visual sensations. This fact has 

 been formulated in the doctrine of specific nerve energy, 

 that different varieties of stimuli, applied to the same organ 

 of sense, always produce the same kind of sensation. And the 

 converse that the same stimulus applied to different organs 

 of sense produces a different kind of sensation for each also 

 holds good. 



The visual mechanism not only gives the power of appre- 

 ciating the degree and source of illumination, but also of 

 appreciating colour. Physically the different colours are 

 simply different rates of vibration of the ether, physiologi- 

 cally they are different sensations produced by different 

 modes of stimulation of the eye. The slowest visible 

 vibrations produce changes accompanied by a sensation 

 which we call red, the most rapid vibrations produce 

 different changes which we call violet. But, as will be 

 afterwards shown, these sensations may be produced by 

 other modes of stimulating the eye. 



The visual mechanism in this way gives a flat picture of 

 the outer world, and from this flat picture we have to form 

 judgments of the size, distance, and thickness of the bodies 

 looked at. 



