AD APT A TION AND IND UCTION. 1 06 1 



surface adjoining a coloured surface, especially when the grey surface is 

 of medium intensity. The contrast effect is much aided by the absence 

 of other differences between the contrasting surfaces. Differences of 

 texture or of glaze, inequalities of surface, and similar details, obscure 

 colour contrast very greatly. Nearly all grey papers are more or less 

 coloured, but the colour may not be obvious on simple inspection, and 

 may only become apparent when a disc made of the paper is rotated. 

 A rapidly-rotating disc presents a uniform surface, which is better 

 adapted for the discrimination of slight coloration. Hering ascribes 

 to the same factor the visibility at a distance of a grease spot on 

 cloth, which is imperceptible when looked at closely. It is for the same 

 reason that colour contrast appears so well on a rotating disc, and that 

 contrast is so much favoured by covering the induced and inducing 

 surfaces with transparent tissue paper. The same factor also accounts 

 for the distinctness of contrast effects in the after-image. If the after- 

 image of Fig. 384 is observed, the two parts of the cross will be found to 

 differ in brightness more than in the original. 



It was formerly believed that contrast occurred most readily with 

 moderate degrees of saturation, and it was supposed that the favour- 

 able effect of covering the contrasting surfaces with tissue paper was 

 due to lessened saturation. Kollett 1 and Hering, 2 on the other hand, 

 found that contrast increased with saturation. The subject has been 

 quantitatively studied by Kirschmann, 3 and by Pretori and Sachs. 4 

 Two chief methods have been used. A surface coloured by contrast 

 may be compared with an objectively coloured field ; or advantage 

 may be taken of the fact that the effect of a saturated surface on a 

 less saturated surface of the same colour is to abolish the colour of 

 the less saturated surface, and the contrast effect may be measured 

 by finding the amount of colour in the less saturated surface which 

 can just be neutralised. The latter method has been found to be the 

 more satisfactory, and the above observers agree in finding that the 

 contrast effect increases with increase of saturation of the inducing 

 surface. According to Kirschmann, the former seemed approximately 

 to increase in arithmetical, when the saturation increased in geometrical, 

 progression. Pretori and Sachs found a definite relation between the 

 white value of the reacting surface and the amount of colour that could 

 be neutralised, the two increasing proportionally. 



The influence of the extent of the contrast inducing surface was also 

 investigated quantitatively by Kirschmann, who used two discs at a 

 certain distance from one another. In the case of brightness contrast, 

 it was found that the contrast effect increased with increase of extent of 

 the inducing surface, the relation between the two being approximately 

 logarithmic. Colour contrast was also found to increase with extent of 

 the inducing surface, and apparently in the same ratio as brightness 

 contrast. 



If two surfaces of different colour adjoin a third suitable surface, 

 the latter may be seen to be coloured by contrast, in the colour which 

 is complementary to that which would be produced by mixture of the 

 two inducing colours. 



1 Sitzungsb. d. k. Alcad. d. IVissensch., Wien, 1867, Bd. lv. Abth. 2, S. 344. 



2 Arch. ' f. d. gcs. Physiol., Bonn, 1887, Bd. xli. S. 1. 



3 Phil. Stud., Leipzig, 1891, Bd. vi. S. 417. 



4 Arch. f. d. gcs. Physiol, Bonn, 1895, Bd. lx. S. 71. 



