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PRINCIPLES OF GENERAL PHYSIOLOGY 



attained under the action of light, which is only kept up by the continuous supply 

 of light energy. (See Bauer, 1911, on the regeneration of visual purple, which 

 continues during the action of light.) 



It may be mentioned that Brossa and Kohlrausch (1913-1914) have found that 

 the form of the electrical response varies with the wave length. The work of 

 Frohlich (1913), also, on the eye of the Cephalopod requires consideration. This eye 

 presents certain advantages as regards the question before us. As already pointed 

 out, the nervous elements are situated in a ganglion outside the eye itself. Although 

 the retina is very highly developed, the visual elements are of one kind only, similar to 

 the rods of the vertebrate retina. It has visual purple and the receptors are exposed 

 directly to the light rays. Frohlich confirms the result of Piper that the electrical 

 response is less complex than that of the vertebrate. But the chief interest of his 

 work consists in the demonstration of the fact that the retinal electrical response 

 is not a steady one, but consists in a series of rhythmic waves, from 20 to 100 

 per second, being more rapid the more intense the illumination. These waves are 

 also to be seen on the return of the curve after cessation of the stimulus, but of a 

 lower rate. There is no indication of a " dark " response in the same direction as 

 that on illumination. The effects of red and of blue light were also compared with 

 that of white and the interesting fact found that red light required to be increased 

 enormously more than white or blue to give the same increase in electrical effect, 

 as shown in the table : 



With the same intensity, the rate of the rhythmical changes is less with red than 

 with blue. Whatever may be the significance of the vibratory nature of the 

 electrical change, it is clear that it does not represent the actual rate of 

 vibration of the light itself, but it does not appear to me that the author's 

 conclusion that the red end of the spectrum is exciting, the blue end inhibitory, 

 on account of the rapid rate of the waves produced, is a necessary one. The 

 view of the Verworn school that rapid rates of nerve impulses produce inhibition 

 has been discussed previously (page 426). 



Colour Vision. This important question cannot be adequately treated here 

 and the reader should refer to the various textbooks. There are, however, some 

 facts, chiefly brought out by the work of Edridge-Green (1909 and 1911), to which 

 brief reference must be made, because they are only just beginning to receive the 

 attention they deserve. 



The Young-Helmholtz theory assumes that there are only three primary colour 

 sensations, red, green, and violet. Now, while it is true that any colour may be 

 formed by mixtures of these in appropriate proportions, it is also true that more 

 than three primary sensations would also serve the same purpose, three is, in 

 fact, the minimum. And it is a matter of universal experience that blue and 

 yellow have just as much right to be considered primary as the other three. In 

 fact, Newton's division of the spectrum into red, orange, yellow, green, blue, indigo, 

 and violet is much nearer the truth. Indigo, however, is rarely seen as a distinct 

 colour. Edridge-Green divides people into classes according to the number of 

 distinct colours they distinguish and shows that there are various degrees of 

 colour blindness according to the number of colours seen in the spectrum. From 

 the point of view of evolution of the colour sense, he points out that it is 

 practically certain that the distinction between different wave lengths, that is, 

 the recognition of a difference between colours, would first show itself at the 

 extremes of the region which is appreciated as light, the region between the 



