156 



VETERINARY PHYSIOLOGY 



green, but which can react to blue and yellow. Only the 

 more central part of the retina is capable of being stimulated 

 by all colours. These zones are not sharply defined, and 

 vary in extent with the size and brisrhtness of the coloured 

 image (fig. 75) (Practical Physiology). 



(2) While the various sensations which we call colour are 

 generally produced by vibrations of different lengths falling 

 on the retina, colour sensations are also produced in various 

 other ways. 



Fig. 75.— Distribution of Colour 

 Sensation in relationship to the 

 surface of the retina {Colour Peri- 

 meter). A indicates the extent 

 of retina stimulated by white and 

 black ; B, the part also capable of 

 stimulation by blue and j^ellow ; 

 and G, the central part capable 

 also of stimulation by red and 

 green. 



Fig. 76. — Disc which, when 

 rotated in a bright light, 

 gives the impression of 

 colours. 



(a) By mechanical stimulation of the retina. By press- 

 ing on the eyeball as far back as possible a yellow 

 ring, or part of a ring, may often be seen {Practical Physi- 

 ology). 



(6) Simple alternation of white and black upon 

 the retina may produce colour sensation, as when 

 a disc of paper marked with lines, as shown in fig. 

 76, is rotated rapidly before the eye (Practical Physi- 

 ology). 



