CONSIDERATIONS ON COLOURS. US 



We here pafs over many particulars, which perfons verfed in 

 the fcience of optics, or habituated to the practical applica- 

 tion of colours, will eafily fupply. Befides, the fubfequent 

 part of the memoir, of which we have undertaken to give an 

 account, will furnifh an opportunity of repeating what is moft 

 ueceflary for underftanding thefe fubjecls. 



After thefe preliminaries the author proceeds to obfervations Contrafts. 

 on contrafts. He employs this word to characterize the effect 

 of the fimultaneous vifion of two fubflances differently co- 

 loured, when brought near together under certain circum- 

 iiances. Contrail: then is here a companion, from which re- 

 sults the fentiment of a certain difference, great or fmali. It is 

 pretty generally known, and painters in particular are well 

 aware, that a coloured fubftance occupying a fpace of little 

 extent, and placed near or furrounded by a given colour, has 

 not the fame appearance as in the neighbourhood of another 

 colour: but whence arifes this difference ? 



Before we attempt to anfwer this queftion, let us make an 

 •elfential diftin&ion. The colours in queftion muft be either 

 homogeneal, that is formed of one fort of rays only; or com- 

 pound, that is formed of a mixture of different rays. 



In the firft cafe, it muft be confefTed, we are ignorant, Contrafts of 



whether the approximation of different fimple colours would fin ? ple colours 



1 r r not yet exa- 



produce any alteration in their refpecVive appearance. As mined. 



we feldom have an opportunity of feeing exhibitions of co- 

 Jour of this kind, and it is not eafy to arrange fuch at will, no 

 experiments have yet been made on their contrafts. The 

 iubjecl, however, is well worth iludying. \ 



As to compound colours, and fuch are almoft all thofe of Its effecl pro- 

 natural or artificial fubflances, as our author (hews in the l uc ^. by ^ b ' 



ftracting from a 

 courfe of his paper, the new colours exhibited by contraft are colour the rays 



always conformable to the tint that would be obtained by ab- a " aIo g° us to 



ftracting from the colour proper to one of the fubftances the w j t h it. 



rays analogous to the colour of the other. 



Thus if" we place on red paper a flip painted orange-colour, Orange on red 



the latter will appear nearly yellow: on the contrary, the appeari yellow > 



fame ftrip placed on yellow paper will appear nearly red. If on yellow, red $ 



we place it on violet paper it will refume a yellowiih tint, on violet, yel- 



but different from the former ; and laftly, on green paper it lo ' 



3 t> " " on green, an- 



ivill appear red, but in a different degree. ether red; 



1 Vol. XII.— October, 1805. I The 



