436 Mr. P, Cooper on Accidental Colours, 



being impressed with the light by which it is surrounded, 

 adopts it as the standard for white, and, consequently, 

 sees light varying from it in the proportion of its different 

 rays, of the colour of that ray which predominates, when 

 compared with the standard to which it is referred. — In 

 the second class, the standard is formed by the surrounding 

 light as before ; but this light, being composed of light, 

 the different parts of which are differently proportioned 

 and differently directed, forms different shadows ; and 

 these shadows must necessarily be complementary, because, 

 when the lights, by which they are respectively formed, 

 are mixed together, they form the light which is referred 

 to as the standard. — In the third class, the standard is 

 formed by the mixed sensation produced by the two eyes 

 under the influence of different impressions, which are, 

 therefore, complementary to each other, with reference to 

 this standard. 



In the first class is included, in addition to some of those 

 already mentioned, the experiment of M. Meusnier ; (21) 

 the standard in this case, is formed by the light admitted 

 through the curtain, which refuses transmission to light of 

 certain colours ; the beam of the sun's light, therefore, 

 which is admitted in its original state by a hole a quarter 

 of an inch in diameter, must appear to an eye thus im- 

 pressed, of the colour or colours which the curtain refuses 

 to transmit. 



The azure colour of the atmosphere is also included in 

 this class. The surrounding light, which forms the stan- 

 dard, is deprived of part of its more refrangible rays, by 

 being reflected from coloured objects in which the less 

 refrangible colours prevail, as well as by the greater 

 depth of atmosphere which it must traverse before it meets 

 the eye, the red light being known to penetrate with 

 greater facility than the other colours ; the direct light of 

 the atmosphere, therefore, appears blue when compared 

 with this reflected light, upon the same principle, that 

 the beam of light, in the preceding experiment, appeared 

 green. 



The second class includes the experiment, in which the 

 light of the sun and the light of a candle fall upon different 

 parts of the same white paper ; and also, the various 



