THE SENSATION OF SIGHT. 236 



colour, which for the most part depend upon imperfectly defined 

 after-images. l This effect is known as successive contrast, and is 

 experienced when the eye passes over a series of coloured objects. 

 But a similar mistake may result from our custom of judging 

 of local colour according to the brightness and colour of the 

 various objects seen at the same time. If these relations happen 

 to be different from what is usual, contrast phenomena ensue. 

 When, for example, objects are seen under two different coloured 

 illuminations, or through two different coloured media (whether 

 real or apparent), these conditions produce what is called simul- 

 taneous contrast. Thus in the experiment described above of 

 coloured shadows thrown by daylight and candle-light, the 

 doubly illuminated surface of the paper being the brightest object 

 seen, gives a false criterion for white. Compared with it, the 

 really white but less bright light of the shadow thrown by the 

 candle looks blue. Moreover, in these curious effects of contrast, 

 we must take into account that differences in sensation which 

 are easily apprehended appear to us greater than those less 

 obvious. Differences of colour which are actually before our 

 eyes are more easily apprehended than those which we only keep 

 in memory, and contrasts between objects which are close to one 

 another in the field of vision are more easily recognised than 

 when they are at a distance. All this contributes to the effect. 

 Indeed, there are a number of subordinate circumstances affect- 

 ing the result which it would be very interesting to follow out 

 in detail, for they throw great light upon the way in which we 

 judge of local colour: but we must not pursue the inquiry fur- 

 ther here. I will only remark that all these effects of contrast 

 are not less interesting for the scientific painter than for the 

 physiologist, since he must often exaggerate the natural pheno- 

 mena of contrast, in order to produce the impression of greater 

 varieties of light and greater fulness of colour than can be 

 actually produced by artificial pigments. 



Here we must leave the theory of the Sensations of Sight. 



1 These after-images have been described as ' accidental images,' positive 

 when of the same colour as the original colour, negative when of the com- 

 plementary colour. TK. 



