and several of their singular appearances. 293 



Thus, when the bright Hght conveyed through a window 

 surrounds the transparent paper, it may increase very sensiblv 

 the splendour of the col(Hir of contrast, or injure it by pro- 

 ducing another slude according to the colours of the bodies 

 subjected to observation. In a word, one mav ahvays re- 

 move this inconvenience by concealing the troublesome 

 objects by a piece of black pasteboard or stuff, or bv look- 

 ing through a blackened tube which confines the field of 

 sight to the extent necessary. 



This knowledge of contrasts may be applied with great 

 advantage to those arts which have a relation to colours. 

 The painter knows that one cannot be placed indifferently 

 in the neighbourhood of another. Bui when one knows 

 the law to which their re-action is subject, one knows bet- 

 ter what must be avoided or done to increase the splendour 

 of the colour which it is necessary to heighten : a successive 

 coinjiarison of them furnishes also valuable indications in 

 regard to their nature or their comjiosition. lliis is what 

 the author himself put in practice with advantaf^e in his 

 manufactory of colours and paper-hangings. 



These considerations in regard to contrasts led him to 

 the examination of a very singular case proposed and treated 

 of by Monge with his usual sagacitv*; namely, the white 

 appearance under which a coloured body is sonietimes seen 

 w hen viewed through a piece of glass of the sai-ne colour ; 

 some uncertainty remained in regard to the circumstances 

 really necessary for producing this effect. The author de- 

 termines them by the help of his particular^experiments, 

 and enumerates those which have a favourable influence, or 

 the contrary. He concludes that, when one experiences 

 .the sensation of whiteness in these cases, it arises merely 

 from the action of contrasts, by which the impression of the 

 colour is lessened or annulled, while that of a certain brioht- 

 ness still exists, and is remarked by the opposition of a 

 greater degree of obscurity. This manner of considering 

 the subject leads to a new definition of whiteness, in which 

 there is certainly nothing repugnant : w/iite to ns h the set}' 

 salinn of light ivhen no pariiciilar colour piedum'mates or h 

 pcrceired. 



In the sub-equent part of his memoir the author employs 

 himself in particular with the coloration of different opake 

 or transparent bodies ; that is to say, ht cftdgavours to dis- 

 cover what are tho-^o luminous rays wUich any coloured 

 body is really f)usceplii)le of reflectiHg or transmitting. 



* Annalts <}c ,C)iimie, torn. lii. 



Ts If is 



