THE SEI^SATIOI!^ OF SIGHT. 267 



that of objects seen through it. This is proved by a 

 number of experiments contrived to illustrate the effects 

 of contrast. If we look through a green veil at a field of 

 snow, although the light reflected from it must really 

 have a greenish tint when it reaches our eyes, yet it 

 appears, on the contrary, of a reddish tint, from the effect 

 of the indirect aftei-image of green. So completely 

 are we able to separate the light which belongs to the 

 transparent medium from that of the objects seen 

 through it.' 



The changes of colour in the two last experiments are 

 known as phenomena of contrast. They consist in mis- 

 takes as to local colour, which for the most part depend 

 upon imperfectly defined after-images.^ 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 phe- 

 nomena 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 shnultaneous 

 contrast. Thus in the experiment described above of 

 coloured shadows thrown by daylight and candle-light, 

 the doubly illuminated surface of the paper being tlie 

 brightest object seen, gives a false criterion for white. Com- 

 pared 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 



* A number of similar experiments will be found described in the 

 author's Handbuch der fhysiologischen Optik, pp. 398-411. 



2 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. — Tb. 



