388 THE BODY AT WORK 



quently met with the defect may be described as due to an 

 absence of the sense of redness, or as an absence of the sense 

 of greenness. The two conditions can be distinguished. But 

 since the eye is not dark for red (although in certain cases 

 vision is very weak for the red end of the spectrum) or dark 

 for green, the abnormality cannot be adequately accounted 

 for on structural grounds. It is not explicable on the hypo- 

 thesis that one of three sets of responsive sense-organs (or 

 nerve-fibres) or photochemical substances is absent from the 

 eye. Again, it is generally agreed that the sensations of white, 

 yellow, and blue of the red-green colour-blind are similar to 

 those of normal persons. This is not in harmony with the 

 theory of the omission from their eyes of one of three pieces of 

 colour-apparatus . 



Professor Hering, of Leipsic, adopting the generally accepted 

 view that light effects chemical changes in substances contained 

 in the retina, to which changes stimulation of nerve-endings is 

 due, formulated a theory of colour- vision which many physio- 

 logists prefer to Young's. He imagines that the retina con- 

 tains three kinds of pigment, each of which is, as he believes 

 all living substance to be, in a constant state of change. It 

 is at the same time being built up and destroyed. Using the 

 terms which connote the opposite directions of metabolism, 

 the pigment is simultaneously undergoing anabolism and 

 katabolism ; the two processes, when the retina is at rest, 

 maintaining equilibrium. When light acts upon either of the 

 substances, it hastens, according to its quality, either the one 

 process or the other ; and the chemical change, whether it be 

 constructive or destructive, stimulates the endings of optic 

 nerves. Hering assumes, therefore, that there are six elemen- 

 tary qualities of visual sensation red, green, yellow, blue, 

 white, black Red, yellow, white are due to anabolism of the 

 visual substances ; green, blue, black are due to their kata- 

 bolism. The installation of yellow amongst the unanalysable 

 colours is a relief to many minds. It is almost impossible to 

 think of yellow as a compounded colour. White also, we feel, 

 is not a compounded colour, despite our knowledge that a prism 

 scatters from it all the hues of the rainbow. Black, many per- 

 sons assert, gives them a definite sensation, and not merely a 

 sense of rest. (Parenthetically, it may be observed that the 



