418 STUDIES IN SPECIAL SENSE PHYSIOLOGY 



" From what has been said the following rules can be deduced : 



" If two colours of equal tone and equal purity differ in bright- 

 ness this is due to a difference in their black-white components. 



" Two colours differing in tone may, notwithstanding equal 

 degrees of purity and equality as regards their black-white com- 

 ponents, differ in brightness. 



" With equality of conditions as to the black-white components, 

 a yellow, a red, or a yellow-red colour is so much the brighter, 

 a blue, a green, or a blue-green so much the darker, the more 

 distinct the colour tone in comparison with the black-white con- 

 stituent." ( 27 ) 



This extract describes what is often called the theory of 

 the " specific brightness " of colours. All that has gone before 

 purports to be a faithful analysis of our visual sensations without 

 reference to any hypothesis whatever. That this is a perfectly 

 legitimate procedure I have already attempted to show : the next 

 step is to translate these facts, or supposed facts, into terms of 

 a physiological hypothesis. 



Such a translation can readily be effected. 



It is supposed that somewhere in the retino-cerebral apparatus, 

 in the infra-conscious sphere, four distinct substances exist. Each 

 of these substances can undergo a building up or anabolic and a 

 breaking down or katabolic change. External stimuli will, depend- 

 ing on their natures, induce either an anabolic or katabolic change 

 in the substances, and these are associated with a definite colour 

 sensation. The building up of the black-white substance corre- 

 sponds to a sensation of blackness, its breaking down to a sensation 

 of whiteness ; anabolism of the red-green substance is associated 

 with green colouration, its katabolism with red colouration ; 

 similarly in the third substance yellow is katabolic in origin, blue 

 anabolic. 



Before discussing these views in detail, I should like to clear 

 up certain popular misunderstandings. Some opponents have 

 asserted or suggested that the facts upon which the theory is 

 based differ in some perverse way from those data which are 

 ordinarily called facts of experiment. This is not the case. The 

 facts the hypothesis attempts to describe are as legitimately 

 objects of inquiry as any others within the purview of physiological 

 science. 



It is further to be noted that the four physiological " sub- 



