CHAP, in.] SIGHT. 99 



We may here remark when the extreme red end of the 

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

 between X 760 and X 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 X 430 to 

 X 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 which 

 thus do not change in hue but only in luminosity correspond 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 sensations are supposed to 

 increase, though probably in this part of the spectrum the white 

 sensation is very feeble, rapidly increasing 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. We must content ourselves with incidental reference to 

 some conclusions, 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. 



763. Variations in Coloiw Vision. Colour- Blindness. Persons 

 differ very much in their power of appreciating and discriminating 

 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 identical 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 (Glasgow) college 

 gown when it was lying on the college grass plot , the colour-blind 

 can tell a cherry among the leaves on a tree much more by its 

 form than by its colour, and when such persons are asked to 

 ' make matches ' between coloured objects, such as skeins of 

 coloured wools, they will put together a red skein and a green 

 skein as being of the same colour. Most colour-blind people more 



