1236 THEORIES OF COLOUR VISION. [BOOK in. 



And other examples of a similar kind might be given. Admit- 

 ting then that the " intrinsic light of the retina " corresponds to 

 a condition of equilibrium of the white-black substance, we may 

 speak of this as the neutral condition on one side of which 

 we have sensations of white and on the other side sensations of 

 black. Such a neutral condition has been spoken of as a " neutral 

 grey," but the word grey is so often associated with a mixture 

 of white and black sensations coexisting at the same time rather 

 than with a neutral condition, that the term seems unsuitable. 

 Many minds find it difficult to realize that the condition of which 

 we are speaking is a true neutral condition, the various degrees of 

 blackness being insignificant compared with the various degrees 

 of intensity of white, and accordingly find it difficult to accept 

 Bering's theory. 



Both theories conform to the conclusion ( 761) that normal 



vision is trichromic in the sense of being made up of three 



factors ; for the three pairs of fundamental sensations of the 



one theory (the two members of each pair being reciprocally 



antagonistic, the positive and negative phase of the same thing), 



play the same part in the equations of mixtures as the three 



primary sensations of the other theory. Indeed it will be found 



on examination that all the results of the mixtures of colours 



are equally explicable on both theories. In comparing the two 



theories, however, especially in reference to the results of mixtures, 



we must bear in mind that "brightness" "or luminosity" does 



not possess the same meaning in the two theories. In the 



Young-Helmholtz theory brightness is dependent on the extent 



to which the primary sensation is excited, on the amount of 



energy expended in the physical substratum, whatever that may 



be, of the primary sensation. The red of the extreme red end of 



the spectrum has a minimum of brightness since the extreme red 



rays excite the red sensation to a minimum and the other two 



sensations hardly or not at all. As we pass blue wards the 



brightness increases, partly because the red sensation is more 



powerfully excited, but also because to the brightness of the red 



sensation there is now added the brightness of the green sensation. 



And the brightness of a saturated yellow, such as that of the 



spectrum, is the sum of the brightnesses of the red and green 



sensations and nothing else ; we neglect for the sake of simplicity 



the minute adjunct of the blue sensation. In Bering's theory 



the case is different. The lack of brightness at the red end 



of the spectrum is due not merely to the feeble development 



of the red sensation, to the feeble (katabolic) excitation of the red 



green substance, but also to the feeble development of the white 



sensation, to the feeble (katabolic) excitation of the white black 



substance ; and the brightness of the yellow of the spectrum is 



due not merely to the large development of the yellow sensation 



but also to the large increase of the white sensation. 



