114 COLOUR VISION 



unmixed green, unmixed blue, and pure violet. The changes are as 

 follow : 



(1) All direct sensation of the fatiguing colour is abolished, not 

 merely from the corresponding part of the spectrum, but also from those 

 regions in which it overlaps other colours. It is rather the sensation 

 of the particular colour that is weakened than the sensitiveness of the 

 eye to a particular part of the spectrum. Thus, in red fatigue, the red 

 end is shortened, but the green appears with full intensity at the 

 D line and reaches almost as far as the C line. In green fatigue the 

 green is replaced by red and blue, which not only meet but overlap. 



(2) The fatiguing colour produces a homochromatic after-effect 

 like a luminous fog, by which the hue of all the other colours is modified 

 if they are relatively weak, but which is unnoticed if they are bright. 

 " A strong light not only fatigues the eye, but dazzles it ; that is to 

 say, the sensation of light persists after the source of light has been 

 withdrawn " (Burch). Each " dazzle-tint " has its own rate of develop- 

 ment and of subsidence, and each is independent of the rest. Red passes 

 oft" first, then green, then blue, and last violet. The positive after-effect 

 of light between the C and D lines is at first red, but it comprises two 

 dazzle-tints — red and green, of which red soon vanishes, leaving green. 

 The dazzle-tint becomes unnoticeable long before the colour sensation 

 is fully restored. 



(3) Fatigue with one colour has no effect upon the intensity of the 

 remaining colour sensations. 



Fatigue with intermediate portions of the spectrum produces inter- 

 mediate effects. Thus fatigue with orange or yellow affects not only 

 the appearance of the orange or yellow, but that of the red and green 

 also ; similarly with the blue-greens and the indigo. 



By fatiguing with red for 30 seconds and then finding pure yellow, 

 fatiguing with E for 30 seconds and then finding the beginning of blue, 

 fatiguing with F for 30 seconds, and then finding the beginning of 

 violet, and again working backwards on the same plan, it has been 

 possible to map out the range of the colour sensations. The averages 

 of 70 people with normal colour vision give the extent of 



red = 760-6 to 551 /x/x. 



green = 593 to 484 „ 



blue = 517 to 443 „ 



violet = 454 to 397-5 „ 



