230 Comid«rations on Colours, 



moir, of which we have undertaken to give an account, 

 wiii afford us an opportunity ot" nieutioning what will be 

 niost necessary for understanding the subject. 



These preliminary observations arc followed by some re- 

 marks on contrasts. The author employs this word to 

 characterize the effect of the simultaneous vision of two 

 substances of different colours when brought together 

 under certain circumstances. I'he contrast here, then, is a 

 comparison, from which there results a sensation of some 

 difference, great or small. It is very generally known, 

 and painters know it well, that a coloured matter which 

 occupies a pretty extensive space, and brought near to, or 

 surrounded by, some other colour, has riot the same appear- 

 ance as when it has near it other colours. But whence 

 does this difference arise ? 



Before we answer this question, let us make an essential 

 distinction. It either relates to homogeneous colours, that 

 is to say, formed of one kind of rays, or to complex colours, 

 arising from a mixture of heterogeneous rays, that is to say, 

 composed of different sorts. 



In regard to the first case, it must be confessed that we 

 are ignorant whether the bringing together different simple 

 colours would produce any alteration in their respective 

 appearance. As one can rarely enjoy the sight of such 

 colorations, and as it is not easy to dispose of them to our 

 wish, no experiments have yet been made on their contrasts. 

 This subject, however, deserves to be studied. 



In regard to the cases of compound colours, (which is 

 that of almost all the natural or artificial bodies, as the au- 

 thor shows in the sequel of his memoir,) the new colours 

 manifested by the contrast are always conformable to the 

 shade which would be obtained by suppressing from the 

 colour proper to one of the bodies, the rays analogous to 

 the colour of the other body. 



Thus, if we place on red paper a small slip of paper 

 painted of an orange colour, it will appear almost yellow. 

 Removed then to yellow paper, the same orange slip will 

 Ix-conve almost red. After this, if it be put upon violet pa- 

 per, it will resume a yellowish shade, but flillerent from the 

 preceding; and, in the l;(st place, applied to green paper 

 it will assume a new droree of a red colour. 



The explanation of these examples according to the pro- 

 posed rule is easy, if we suppc^se that the orange colour of 

 the small slip observed arises (as is commonly the case) 

 .from an union of all the kiiuKof rays except the blues. 



A multitude of combiiiaiious of colours placed thus 



above 



