132 



VETERINARY PHYSIOLOGY 



on the retina, colour sensations are also produced in various 

 other ways. 



(a) By mechanical stimulation of the retina. By pressing 

 on the eyeball as far back as possible a yellow ring, or part 

 of a ring, may often be seen. (Practical Physiology.) 



(b) Simple alternation of white and black upon the retina 

 may produce colour sensation, as when a disc of paper 



marked with lines is rotated 

 rapidly before the eye. (Prac- 

 tical Physiology.) 



3. By allowing different parts 

 of the spectrum to fall upon 

 the eye at the same time, it is 

 possible to produce either a 

 sensation of white or of some 

 other part of the spectrum. 

 (Practical Physiology. ) To pro- 

 duce a sensation of white from 

 two or three different parts of 

 the spectrum, a due proportion 

 of each part must be taken, 

 since different parts have differ- 

 ent sensational activity. This 

 may be represented by a plot- 

 ting out the various parts of 

 the spectrum on a curve, and 

 joining them to a central spot 

 by means of lines. The length of the line then represents the 

 relative sensitive activity of the particular part of the spectrum 

 to which it passes (fig. 63). 



This means that by different modes of stimulation of the 

 retina the same sensation may be produced. The sensation 

 of orange may be produced either when vibrations at about 

 580 billions per second fall on the eye, or when two sets of 

 vibrations, one about 640 and one about 560 billions, reach it. 

 By no possible physical combination of the two is it possible to 

 produce the intermediate rate of vibration. 



The sensation of colour, therefore, depends upon the nature 

 of the change set up in the retina, and not upon the condition 

 producing that change. 



FIG. 62. Distribution of Colour Sen- 

 sation in relationship to tlie surface 

 of the retina (Colour Perimeter). A 

 indicates the extent of retina stimu- 

 lated by white and black ; B, the 

 part also capable of stimulation liy 

 blue and yellow ; and C, the central 

 part capable also of stimulation by 

 red and green. 



