BREWSTER'S ANALYSIS OP LIGHT. 



and it consequently has a yellowish tint. The succeeding divisions 

 downwards towards a a become more and more red until they 

 attain the pure prismatic red of the lowest division. The colours 

 of the upper extremity of the image may in like manner be shown 

 to be as follows. 



Between s s and r r v 



rr qq v + I 



,, q q pz> v + i + B 



,, pp oo V + I + B + G 



00 11 n V + I + B-t-G+Y 



n n 

 m m 



mm V+I+B+G+Y+O 



Thus it appears that the highest fringe at the upper edge is 

 violet, that those which succeed it are formed by the mixture of 

 violet and blue, to which green and yellow are successively added, 

 until the colours become so completely combined that the fringe 

 is scarcely distinguishable from a pure white. It is evident, there- 

 fore, that at the lower extremity the reds, and at the upper the 

 blues, prevail. 



If the object A A M M viewed through the prism be not white, 

 then the preceding conclusions must be modified according to the 

 analysis of its colour. Thus, if its colour be a green, it may be 

 either a pure homogeneous green, or one formed by the combina- 

 tion of blue and yellow or other prismatic tints. In" the former 

 case, the prism will exhibit the object without fringes, but in the 

 latter it will be fringed according to the composition of its 

 colour, determined by the same principles as those which have 

 been applied to the object A A M M. 



28. In all that precedes it has been assumed that the light com- 

 posing each part of the prismatic spectrum is simple and homo- 

 geneous. This conclusion, deduced by Newton, and adopted 

 generally by all physical investigators since his time, is based 

 on the assumption, that light which, being refracted by transparent 

 media, cannot be resolved into parts differently refrangible, is 

 simple and homogeneous. 



Sir David Brewster, has, however, published the results of a 

 series of observations, from which it would follow, that a pencil 

 of light which does not consist of parts differently refrangible, 

 may, nevertheless, be resolved into parts which have different 

 colours ; in other words that the light of certain parts of the 

 spectrum, such, for example, as orange and green, although 

 simple so far as respects refraction, is compound so far as re- 

 spects colour. Thus, the orange light may be resolved into two 

 lights equally refrangible, but differing in colour, one being red 

 and the other yellow ; and the green light may in like manner bo 



77 



