24 PSYCHOLOGY. 



the field wliicli surrounds it. Helmholtz explains the fact 

 thus : 



" The difference of the compared colors appears greater than it really 

 is either because this difference, when it is the only existing one and 

 draws the attention to itself alone, makes a stronger impression than 

 when it is one among many, or because the different colors of the sur- 

 face are conceived as alterations of the one ground-color of the surface 

 such as might arise through shadows falling on it, through colored 

 reflexes, or through mixture with colored paint or dust. In truth, to 

 produce an objectively gray spot on a green surface, a reddish coloring 

 would be necessary." * 



This explanation is easily proved false by painting the 

 disk with narrow green and gray concentric rings, and giv- 

 ing each a different saturation. The contrast appears 

 though there is no ground-color, and no longer a single dif- 

 ference, but many. The facts which Helmholtz brings for- 

 ward in support of his theory are also easily turned against 

 him. He asserts that if the color of the ground is too in- 

 tense, or if the gray ring is bordered by black circles, the 

 contrast becomes weaker ; that no contrast appears on a 

 white scrap held over the colored field ; and that the gray 

 ring when compared with such scrap loses its contrast-color 

 either wholly or in part. Hering points out the inaccuracy 

 of all these claims. Under favorable conditions it is impos- 

 sible to make the contrast disappear by means of black en- 

 closing lines, although they naturally form a disturbing 

 element ; increase in the saturation of the field, if disturb- 

 ance through increasing brightness-contrast is to be avoid- 

 ed, demands a darker gray field, on which contrast-colors 

 are less easily perceived ; and careful use of the white scrap 

 leads to entirely different results. The contrast-color does 

 appear upon it when it is first placed above the colored 

 field; but if it is carefully fixated, the contrast-color di- 

 minishes very rapidly both on it and on the ring, from causes 

 already explained. To secure accurate observation, all 

 complication through successive contrast should be avoided 

 thus : first arrange the white scrap, then interpose a gray 

 screen between it and the disk, rest the eye, set the wheel 

 in motion, fixate the scrap, and then have the screen re- 



* Helmholtz, loc. cit. p. 412. 



