SUCCESSIVE INDUCTION OK AFTER-IMAGES 109 



stimulus plus the white light of the white or grey surface. If it were, 

 after stimulation with blue, for example, the yellow of the after-image 

 would always be compensated by mixing the same amount of blue 

 with the secondary light, so that a white after-image would result. 

 By the coefficient law, with increasing intensity of the white light 

 the amount of blue added should increase proportionately, and this 

 is found to be the case (v. Kries). The complementary after-image is 

 therefore not the result of a mere additive process, but is a genuine 

 qualitative change in the white light, though it may also involve a 

 change of intensity. 



If the primary stimulus is coloured and the secondary stimulus also 

 coloured various coloured after-images are obtained. It has already 

 been mentioned that if the secondary stimulus is the complementary 

 colour of the primary the resulting sensation is that of the extremely 

 saturated complementary colour. This is specially true of the red- 

 yellow end of the spectrum and for secondary lights of high intensity. 

 It may indeed be far more saturated than any spectral colour, so that 

 it is impossible to obtain a comparison light which will match it. 

 This is a fact of profound theoretical significance, since it shows that 

 we are capable of experiencing visual sensations which cannot be 

 elicited by the application of any known physical stimulus to the 

 resting eye. 



If the secondary stimulus differs from both the primary and its 

 complementary the most diverse results may be obtained, but as a 

 rule the complementary colour of the primary stimulus predominates. 

 After green yellow appears reddish orange, blue purple, and so on. 

 After red, yellow (589 /xyu) matched greenish-yellow (556 fi^) ; after 

 green, 517 fi/u, matched 565 /^/z (Hess^). 



Attempts have been made by v. Kries^ for white, and by Schon^ 

 for coloured lights, to obtain the time relations of after-images. The 

 sensation derived from the secondary light is matched at rapid intervals 

 with a comparison light. The results are of little value owing to rapidly 

 changing adaptation and the difficulty of avoiding parafoveal stimula- 

 tion. V. Kries* also studied the time relations of recovery from the 

 primary stimulus. It is unknown whether the effect passes off as a 

 smooth curve (Fechner, v. Helmholtz, v. Kries) or in rhythmical waves 



1 Arch. f. OpMh. xxxix. 2, 45, 1893. Cf. A. W. Porter and Edridge-Green, Proc. Roy. 

 Soc. Lond. B. lxxxv. 434, 1912; Edridge-Green, loc. cii. lxxxvi. 110, 1913. 

 3 Arch.f. Ophth. xxni. 2, 1, 1877. 3 Ibid. xx. 2, 273, 1874. 



* loc. cit. 



