112 COLOUR VISION 



after-images. As the former passes into the latter in the case of a strong 

 primary stimulus the image does not simply become paler, but un- 

 saturated hues appear. The homochromatic hue of the positive image 

 first disappears and the image resembles that of the after-image of a 

 white object in which the rose-red phase predominates. " Then the 

 complementary colour of the negative after-image gradually developes, 

 but it may be already visible before the positive image has become 

 negative ; it may therefore appear brighter than the dark background. 

 I think that the appearance of the complementary colour may be 

 attributed to the fact that the positive image, which at this phase has 

 become feeble and white, is superposed upon the negative and comple- 

 mentary images induced in the intrinsic light chaos by the fatigue of 

 the eye. It is clear that by such a superposition after exposure to red 

 the positive white and the negative blue-green can together give a 

 greenish white positive image. These positive complementary images 

 are mentioned by several observers (Purkinje 1 , Fechner 2 , Briicke 3 ) 4 ." 

 It is an instructive experiment to view the spectrum momentarily and 

 then to observe the phases of the after-images. 



Burch 5 has adduced strong evidence to show that the duration of 

 after-image effects is much longer than has been commonly supposed. 

 He has observed changes occurring in the subjective sensations for as 

 long as two hours. As with all subjective observations of this kind 

 generalisations from them are to be accepted with caution. 



It is unnecessary to pursue this complicated problem further. 

 The older experiments, especially Fechner's with coloured stimuli, were 

 mostly made with impure lights. The matter requires further investiga- 

 tion with homogeneous lights and due regard for adaptation, etc. 



CHAPTER III 



THE EFFECTS OF " FATIGUE ' 



The term " fatigue " has been avoided in the previous consideration 

 of the effects of retinal stimulation. With our increased knowledge of 

 adaptation it has become difficult to estimate the importance of fatigue 

 and further discussion must be postponed until we deal with the theory 



1 Zur Physiol. d. Sinne, n. 110. 1819. 2 Ann. d. Physik, L. 213, 1840. 



3 Wiener Denkschr. m. 12, 1850. 4 v. Hclmholtz, loc. cit. n. 213. 



5 Proc. Roy. Soc. Lond. B, LXXVI. 212, 1905. 



