SUCCESSIVE INDUCTION OR AFTER-IMAGES 105 



fixated for more than a few seconds, gives rise to an after-image sur- 

 rounded by a halo. If the coloured light is not very bright, and the 

 fixation not very prolonged, the halo is of the same colour. The longer 

 fixation is continued, the brighter and the less saturated is the colour of 

 the halo, until after prolonged fixation it becomes white or even tinged 

 with the complementary colour, and is so much brighter than the after- 

 image itself as to inhibit it partially or wholly. The colours of the 

 after-image and its halo tend to be complementary to one another, 

 for during the observation of an after-image in the dark the conditions 

 are very favourable to contrast effects (See Section VI). 



The effects of a secondary stimulus on a previously stimulated 

 area are of great importance in considering the facts of colour vision. 

 We have seen that the primary stimulus alters the excitability of the 

 retina so that it is lowered for a succeeding stimulus of the same nature. 

 So far at any rate as colours are concerned we may carry the matter 

 further and say that it is raised for a secondary stimulus of the com- 

 plementary colour. If a complementary after-image of any spectral 

 colour be obtained and the secondary stimulus be this complementary 

 itself, a sensation of this complementary colour is produced which far 

 exceeds in purity and saturation any such colour which is found in 

 nature or in the spectrum. If for instance a complementary after- 

 image of green obtained from a suitable purple field be compared with 

 a patch of the corresponding spectral green, it will appear of extra- 

 ordinary brilliancy and saturation. 



The question therefore arises whether these after-effects seriously 

 complicate the equations of colour matches. It may be said at once 

 that so far as the peripheral retina is concerned they do, because we 

 have already seen that the periphery values are not the same as the 

 foveal values and they are far more susceptible to variations in adapta- 

 tion. With regard to the fovea, however, it is found that all colour 

 matches still remain valid, no matter what kind of light may have 

 previously stimulated the retina. Thus a match of monochromatic 

 yellow with a mixture of red and green remains a match after 

 previous illumination with yellow and blue. If yellow has been 

 used, both become paler ; if blue, both become more saturated 

 yellow ; but they still match. This law has been systematically in- 

 vestigated by Biihler 1 under v. Kries' direction 2 . It may be stated 



1 Diss. Freiburg, 1903. 



2 See also v. Kries, Arch. f. Anat. 503, 1878 ; and Dittler and Orbeli, Arch. f. d. ges. 

 Physiol. cxxxn. 338, 1910. 



