CONSIDERATIONS ON COLOURS. JJ7 



when we have the perception of whitenefs in thefe cafes, it is 

 owing folely to the action of contrafh, by which the impref- 

 dion of the colour is deadened or annihilated ; while that of a 

 certain degree of brightnefs (till fubiiOs, and is noticed from 

 the oppofition of a greater degree of obicurity. This manner New definition 

 of confidering the fubjeCt leads to a new definition of whitenefs * whitenefs. 

 which has certainly nothing in it incontinent : white is ivith 

 re/pecl to us the Jen/at ion of light, ivhen no particular colour pre- 

 dominates in it, or is perceived in it. 



In the fubfequent part of his memoir our author particularly Further fubjeijfc 

 coniiders the colouring of different opake and transparent o{ in< l uii y* 

 bodies; that is to fay, he inquires what are the luminous rays 

 which a given coloured body is really capable of reflecting 

 or transmitting. 



His method of making his experiments is fimple. If the Method of ~ 



fubftance be opake, he places it on a piece of black cloth, makli }S h ' s 



. . . . experiments* 



and oblerves it with the prifm. If it cannot be cut fo as to 



reduce it to a re&angular figure, he covers it with a piece of 

 blackened pafteboard, in which there is a rectangular aper- 

 ture. Under thefe circumftances the coloured fringes dif- 

 p*Iayed on two opposite fides indicate the kind of rays reflect- 

 ed, and confequently thole abforbed when we know the 

 nature of the illuminating pencil. On which we have farther 

 to remark, that, as the fringes are themfelves compound tints, 

 the fimple tints that compo.fr them muff, be difcnminated. 

 Their infpe<5tion fuffices an experienced pcrfon for this; but Compound tints 

 the habit is to be acquired, and its place fupplied, by taking t0 b ^ dlfcnmi - 

 for a guide papers reprefenti ng each fpecies of rays, placing How. 

 them in their order one upon another, and drawing them back 

 in gradation conformably to their difference of refrangibility : 

 or we may ufe a table conftructed after Newton's method for 

 determining the compound tints of feveral elementary colours. 



If the body to be examined be traniparent, the aperture in Method of 

 the pafleboard j uft mentioned will be well adapted to cover it examinin g tran * 

 when placed againlt the light, fo that the pnfm may, exhibit 

 fringes on it. Or, if the obferver place himlelf in the dark, a 

 light, as that of a candle, will exhibit through the tranfparent 

 -fubftance, by the aliiftance of the pril'm, a fenes of coloured 

 images correfponding to the ray's tranfmitted. 



Making his experiments in this manner, our author difcover r Colours of 

 v cd that feveral opake fubftances which happened to be at opake bodie * 



hand, forption. 



