108 COLOUR VISION 



the exposure to white changes the chromatic stimulus values. Thus, 

 using a comparison light, and experimenting with rotating discs v. Kries 1 

 found that if the coloured sectors were made the same both for the 

 stimulated and the comparison area there was never even an approximate 

 match ; and if, keeping the same coloured sectors, the white sectors 

 were chosen such that the luminosities were equal, the colours seen by 

 the stimulated area were of far too low saturation. In order to produce 

 a match it was necessary for the amounts of coloured light in the 

 secondary and comparison to be about 3 : 1 (blue sectors 270 : 97 ; red, 

 270 : 84 ; yellow, 270 : 97). These experiments have been adversely 

 criticised by Bering 2 and diverse results have been obtained by Dittler 

 and Richter 3 . 



Dittler and Richter arranged that two contiguous areas of the 

 retina were stimulated with homogeneous blue lights so that an exact 

 match was obtained. The stimulus was then cut off from one area and 

 the other was stimulated with a binary white light obtained by mixing 

 the blue with its complementary colour. The previous blue matches 

 were then replaced and it was now found that the blue of the stimu- 

 lated area was much more saturated than that of the resting area. 

 Increase of intensity of the blue of the stimulated area failed to re- 

 establish a match, but addition of white light effected a much better 

 match. 



// the 'primary stimulus is coloured and the secondary stimulus 

 white the chromatic excitation causes the white to be coloured 

 with the complementary colour. This is the ordinary complementary 

 after-image, a red, yellow, green, or blue object giving a blue-green, 

 blue, purple, or yellow after-image respectively when a white or grey 

 field is fixed. If the sensation is matched with a comparison light on a 

 neighbouring retinal area it can be shown that it is nearly if not quite 

 as saturated as the spectral colour, even if the exciting light is of 

 moderate intensity and has acted for only 30 to 40 seconds. Exact 

 coincidence with the complementary colour of the exciting light is not 

 generally found 4 . 



The colour of the resultant sensation is not a mixture of the colour 

 which would be observed with the darkened eye without any secondary 



1 Berichle d. Freiburger Naturf. Gesellschaft, 1894. 



2 Arch. f. (/. ges. Physiol. xciv. 533, 1903. 



3 Ztsch. /. Sinncsphysiol. XLV. 1, 1910; Dittler and Orbeli, Arch. f. d. ges. Physiol 

 cxxxn. 338, 1910. 



* Cf. Tschermak, Ergeb. d. Physiol. n. 2, 763, 1903. 



