116 



CONSIDER ATIONS ON COLOURS. 



Refleftcd light 

 rauft be avoided 



pound of all the coloured rays, the red rays for example, th« 

 remaining pencil ought to appear a very pale blueifh green-: 

 but, as in the experiment above the little white flip is in the 

 (hade, the black hence arifing may be of a proper degree to 

 deftroy the effect of the white, and then the blueifli green ap- 

 pears of a lively tint. The fame reafoning is applicable to the 

 cafe of" all the other colours. 



To obtain the full effect in repeating thefe experiments, we 

 mutt lake care, while procuring a favourable light, to guard 

 againft the reflection oT adjacent bodies, and againft double 

 coloured fringes. Thus when the bright light tranfmitted 

 through the window furrounds the tranfparent paper, it may 

 very fenfibly augment the bright nei* of the colour of contrail, 

 or injure it by introducing another tint, according to the colour 

 of the body under observation. We have it always in our 

 power, however, to get rid of this fupercompofition, by taking 

 a piece of black cloth or pafteboard to malk the object thus 

 incommoded, or by looking through a blackened tube fo as to 

 confine the field of vifion to the neceffary extent. 



This knowledge of contrail may be ufefully applied to thofe 



arts, which are employed on the fubject of colours. The 



painter is aware, that it is not a matter of indifference what 



colour is placed near another: but when he is acquainted 



with the law, to which their action on each other is fubjected, 



he will know better what to avoid, and how to difpofe his tints, 



fo as to heighten the brilliancy of that which he withes to bring 



forward. Contrafling them together in fuccefiion likewife 



affords us valuable indications of their nature and composition. 



This the author himfelf has put in practice with advantage in 



his manufactory of colours and paper-hangings. . 



White appear- Thefe confederations on contrails led him to the examination 



anceofaco- f a ver f 1P g U | ar cafe, which Mr. Monge has mentioned and 



loured body J . . ... 



through glafs of treated with his ufual fagacity *. This ca(e is the white ap- 



the fame hue. pearance, which a coloured body fometimes exhibits when 

 viewed through a glafs of the fame hue. There remained 

 fome uncertainty relpecling the circumftances actually neceffary 

 for producing this effect : thefe our author determines by par- 

 ticular experiments, and he enumerates thole which have a 

 favourable influence or the contrary. His conclufion is, that. 



How, 



Ufeful in the 



arts. 



* Annaki dc Cktmie, Vol. III. 



whea 



