ON COLOUR-BLINDNESS. 



199 



appeared, however, that a I'od object hold close to the eye 

 was recognised as coloured, but at a distance the shade was 

 simply dark. It matched with a deep grey. I think that 

 this eye saw red at a distance merely as non-illuminated, 

 and that a red object among green was invisible, simply 

 owing to its appearing dark and like the deep shadows of 

 the green. I had no opportunity of testing thi.s case with 

 a spectroscope. 



The most striking confutation, it appears to me, of the 

 theory that colour-blindness is owing to tlie absence of pei'- 

 oeption of one or more of the so-called primary colours, is 

 furnished by the case of a celebrated engraver, who was 

 absolutely incapable of colour-perception of any kind. Now 

 if the perception of colour depends on the existence of three 

 sots of colour nerves, and if all three are inactive, it surely 

 folU)ws that light itself will not be perceived. But if in 

 such an abnormal case the eye is sensitive to light of one 

 period of vibration only, the effect will bo that monochro- 

 matic light and total colour-blindness will result. 



It is, of course, always conceivable that the particular 

 defect which causes colour-blindness lies in the brain, and 

 not in the eye. Certain persons are incapable of judging 

 which of two musical tones is the higher, even when they 

 are more than an octave apart. Yet suoli persons hoai' either 

 tone perfectly ; the defect is not one of deafness. It must 

 be concluded that in such a case the brain is the defaulter. 

 And it may equally well bo the case that the inability to 

 perceive certain colours is not due to a defect in the instru- 

 ment of sight — the eye, but to the power of interpreting 

 the impressions conveyed to the brain by the optic nerve. 

 If this is the case, the problem is no longer a physical one, 

 it falls among those with which the mental physiologist has 

 to deal. 



