98 THEORIES OF COLOUR VISION. [BOOK in. 



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

 ting tlu-n that the "intrinsic light of the retina" corresponds t<> 

 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 

 Hering'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 bluewards 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 Hering'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. 



