COLOR VISION 883 



another quality, and a piece of red paper held against a red background, 

 does not appear nearly so saturated as one contrasted, say, against 

 white. A similar contrast may be obtained by rotating a white disc 

 containing a certain amount of black, as illustrated in Fig. 481. On 

 rotation this disc ought to yield uniform circles of gray, their bright- 

 ness being least in the center. Instead, each circle presents a darker 

 outer and lighter inner margin, because the former borders on a zone 

 darker than itself, while the latter borders upon a zone lighter than 

 itself. 



These phenomena of contrast may also be extended to colors. 

 Thus, if a piece of gray paper is placed upon a larger green sheet, the 

 former appears pink or rose-red. The intensity of the latter color may 

 be increased by covering the whole with a sheet of tissue-paper. It 

 may also be illustrated by the approximation of colored shadows. 



A B 



FIG. 481. A, BLACK AND WHITE Disc FOR EXPERIMENT ON CONTRAST; B, SHOWING THE 

 RESULT WHEN THE Disc A is SET INTO RAPID ROTATION. (Rood.) 



This can be done by placing an object of suitable size and shape upon 

 a white background and illuminating it from one side with day-light 

 and from the other with gas-light. Two shadows result which are 

 sharply contrasted against one another. The one thrown by the gas- 

 light appears yellow, while the one produced by the day-light exhibits 

 a bluish tint, for the reason that it is contrasted against the general 

 yellowish illumination. 



The hypothesis of Helmholtz which refers contrast to an erroneous 

 judgment, has been severely criticized by Hering who holds that this 

 phenomenon is due to the opposing influences of two different regions 

 of the retina and the visual association areas corresponding to them. 

 Hering, therefore, ascribes them to the peripheral part of the visual 

 mechanism and removes from them any purely psychic character. 

 Evidently, he imagines them to be opposing processes of assimilation 

 and dissimilation, similar to those occurring during color vision. This 

 implies that while a dissimilation of a particular substance may be 

 going on in one part of the retina, a neighboring area may show assim- 

 ilation. McDougall compares these phenomena of contrast to the 



