and several of their singular Appearances. £91 



above each other produce the colour of contrast indicated 

 by the rule here given ; but there are several circumstances 

 which render the effect of it more striking, or modify the 

 result. 



It sometimes depends on the degree of the brightness 

 with which the observed bodies are affected : they may be 

 uniformly illiuninated, or one of them more than the rest. 

 The quantity of light which has entered simultaneously into 

 the eye by the whole field of vision has also an influence. 

 If the bodies consist of several rows, like a series of decreas- 

 ing circles placed one within the other, the colours of each 

 will re-act respectively on each other. At each junction 

 there will be on both sides a border coloured by the con- 

 trast of the neighbouring body : these borders will extend 

 more or less according to the splendour of the objects. The 

 effect of one may become dull, or extinguish all the rest. 



The colours of contrast show themselves also with more 

 vivacity after some moments of observation, or if the ob- 

 jects have been agitated a little, as if to make them move 

 slowly over the retina. It would appear that a certain fa- 

 tigue of the eye, either instantaneous in regard to the inten- 

 sity of the light, or more slowly by prolonged vision, con- 

 curs to produce the appearances in question. But exces- 

 sive fatigue of that organ would occasion a degeneration of 

 the colours belonging to another mode. 



We ought not, then, to refer to contrasts those impres- 

 sions mentioned by Epinus, which are propagated in the 

 eye with a certain duration and a particular period of shades, 

 when one has looked with intensity at a very brilliant light, 

 such as that of the sun. 



But the colours called by Buffon accidental, and respect- 

 ing which Scherfer has given an interesting memoir, belong 

 to the class of contrasts, or at least constantly follow the 

 ?ame law. 



Coloured shadmvs are also a phsenomenon of the same 

 kind. Count Rumford has placed this truth beyond all 

 doubt in two memoirs, in which he has treated this subject 

 in an interesting manner*. 



The author of that which wc here analyse is of opinion, 

 that v/e must ascribe also to contrasts those appearances of 

 the solar light received through a hole in a c&lcureu cur- 

 tain, which general Meusnier remarked in consequence oi 

 their singularity. He assimilates to this also several cases 

 of colours exhibited by opals, or more generally by bodies 



• Philosophical Kisavs, vol. i. p. 31? et M-q. 1802, London edit. 



T %; containing 



