906 COLOUR-BLINDNESS. [Book hi. 



many statements regarding the results of mixing sensations, 

 such as those respecting complementary colours (§ 562) have to 

 be qualified by considerations touching the luminosity of the 

 constituents; but into these questions we cannot enter here. 



We may here remark when the extreme red end of the 

 spectrum is examined it is found that along a certain length, 

 between A. 760 and A, 655, there is no change in the sensation 

 as regards hue but only as regards luminosity ; the red remains 

 exactly the same kind of red, it only becomes brighter and 

 more readily seen. Similarly at the other end from A 430 to 

 A 390 the sensation of violet remains of the same hue though 

 differing in luminosity. And these facts have been brought 

 forward on the Young-Helmholtz theory in support of violet 

 being a primary sensation ; it is urged that the red and violet 

 winch thus do not change in hue but only in luminosity cor- 

 respond to the actual primary sensations. The behaviour at 

 the red end is quite intelligible on Hering's theory, since, as 

 the waves shorten in length both the red and the white sensa- 

 tions are supposed to increase, though probably in this part of 

 the spectrum the white sensation is very feeble, rapidly increas- 

 ing a little farther on. The behaviour at the violet end presents 

 difficulties, since if the violet be due to admixture with a second 

 octave so to speak of red, the violet should change in hue, 

 become more red, as the rays shorten. But the same difficulty 

 presents itself to the Young-Helmholtz theory if blue be 

 accepted as a primary sensation. Moreover observations on 

 this part of the spectrum are exceedingly difficult. We cannot, 

 however, attempt to discuss the contending theories properly ; 

 this would carry us beyond the limits of this book. AVe 

 must content ourselves with incidental reference to some con- 

 clusions, which are suggested by the study of some other 

 features of colour sensations as well as of abnormal colour 

 vision, and to these we may now turn. 



§ 566. Variations in Colour Vision. Colour-Blindness. Per- 

 sons differ very much in their power of appreciating and dis- 

 criminating colours, and that quite independently of their ability 

 to give expression to their colour sensations, that is to say, of 

 their skill in naming colours. One person will regard as iden- 

 tical two colours which another person recognizes as different. 

 In many cases such differences in the power of discriminating 

 colours are slight, but in some cases they are great. Certain 

 persons are met with who regard as quite alike, or nearly alike, 

 colours which to most people are glaringly distinct ; such 

 persons are said to be "-colour-blind." 



The most common token of " colour-blindness " is the 

 inability to distinguish, or the difficulty in distinguishing, red 

 and green. The great chemist Dalton, who was colour-blind, 

 found great difficulty in recognizing at a distance his red 



