u8 VETERINARY PHYSIOLOGY 



brain. 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 sensation. This fact has been for- 

 mulated 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, physiologically they are 

 different kinds of 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 judgments are formed 

 of the size, distance, and thickness of the bodies looked at. 



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

 stimulated by the light coming from the object. If a large 

 surface is acted upon, the object seems large ; if a small 

 surface, the object seems small. But the extent of eye-cells 

 acted on depends not merely upon the size of the object, but 

 also upon its distance from the eye, since the further the object 

 is from the eye the smaller is the image formed. Hence, our 

 ideas of size are judgments based upon the size of the picture 

 in the eye, and the appreciation of the distance of the object. 

 The distance of an object, when over fifty or sixty metres from 

 the eye, and very probably even when over as little as six 

 metres, is judged by the modifications in its shading and colour 

 due to the condition of the atmosphere. A range of hills will 

 at one time be judged to be quite near, at another time to be 



