CHAP, in.] SIGHT. 905 



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 ( 564) that nor- 

 mal 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 " lumi- 

 nosity " 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 sen- 

 sation is more powerfully excited, but also because to the bright- 

 ness 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 bright- 

 ness 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 devel- 

 opment of the yellow sensation but also to the large increase of 

 the white sensation. When, moreover, we come to examine 

 this feature of 'brightness' or "luminosity" more closely, we 

 find that many questions of great complexity are raised; and 



