138 VETERINARY PHYSIOLOGY 



illuminated. From the previous training of the nervous 

 system we have been 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. 

 Instead of actually looking at a corner we may be looking 

 at the picture of one. 



From the very lirst it must be remembered that the 

 modification of our consciousness which we call vision is not 

 directly due to external conditions, but is a result of changes 

 set up in our brain. Our sensation is associated with 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 

 chansfes in consciousness — visual sensation. It was in 

 connection with vision that Johannes Miiller formulated the 

 doctrine of specific nerve energy, that different varieties of 

 stimuli applied to the same organ of sense always produce 

 the same hind of sensation. The converse also holds good, 

 that the same stimulus applied to different organs of sense 

 p)roduces a different hind of sensation for each. 



4. The visual mechanism gives the power not only 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 ; physio- 

 logically they are different kinds of changes set up in the 

 retina ; psychologically they are different kinds of sensa- 

 tions. The slowest perceptible vibrations produce changes 

 accompanied by a sensation which we call red, the most 

 rapid vibrations produce a different set of changes which we 

 call violet. But, as will be afterwards shown, these sensa- 

 tions may be produced by other modes of stimulating the eye. 



A flat picture of the outer world is formed, and, from this 

 flat picture, we have to make judgments of the size, distance, 

 and thickness of the bodies looked at. 



The idea of size is based upon the extent of the eye-cells 



