110 COLOUR VISION 



(Plateau, Hess^). From analogy the latter is the more probable, but 

 the question requires further elucidation^. 



The fading of after-images. The observations recorded above deal 

 with successive induction resulting from short exposures. There are 

 a number of careful observations by some of the older authors, 

 notably Plateau^, Fechner*, Seguin^ and v. Helmholtz^, which are of 

 importance in the theory of colour vision. They deal with the subjective 

 impressions which occur during the fading of the after-images of a light 

 stimulus. 



If the primary stimulus be of sufficient intensity the passage of the 

 positive into the negative or complementary after-image is accompanied 

 by a series of colour sensations. Of particular interest for our present 

 purpose is the subjective development of colour sensations from stimulation 

 with white light. The series varies according to the intensity and 

 duration of the primary stimulus. Momentary excitation of white 

 light produces an after-image which " passes rapidly through greenish 

 blue (green, Seguin) into beautiful indigo blue, later into violet or 

 rose-red. These colours are bright and clear. Then follows a dirty 

 or grey orange, during which the positive after-image generally changes 

 to a negative, and in the negative image this orange often becomes 

 a dirty yellow-green'^." When the negative after-image has developed 

 it appears dark against the background of the " intrinsic light " 

 [v. p. 104). If white light of low intensity is now admitted the after- 

 image passes on into the later coloured phases, and retreats again if 

 the white light is again diminished. Thus, if the image when the eye 

 is covered is blue the introduction of dim light causes it to pass through 

 rose-red to a negative yellow. If the eye is again quickly covered 

 the blue reappears. If the image in darkness is rose-red, a weak white 

 light turns it yellowish-red, and so on. If the positive image has com- 

 pletely disappeared from the darkened eye a dim white light develops 

 a grey or green-grey negative after-image surrounded by a rose-red 

 halo. 



If the primary stimulus is more intense or acts for a longer time 

 colouration may commence before obscuring the eye (Fechner). It is 

 first yellow, then blue-grey or blue, without passing through green, 



1 Arch. /. (I. (jes. Physiol, ci. 226, 1904. 



' Cf. Fick and Gurber, Arch. f. Ophth. xxxvi. 2, 245, 1890; Fick, loc. cit. xxxvra. L 

 118, 1892 ; Hering, loc. cit. xxxvra. 2, 252, 1892; Fick, loc. cit. xxxviii. 3, 300, 1892. 

 3 Essai dfune Theorie gen. etc. Bruxelles, ] 834. * Ann. d. Physik, l. 220, 1840. 



* Ann. de Chimie, 3rd series, xli. 415, 1850. 



• 3rd ed. n. p. 208. '^ v. Helmlioltz, loc. cit. ii. p. 208. 



