I 



1835.] Composition of White Light. 113 



Yellow light may be readily obtained by transmitting 

 white light through a yellow glass, slightly inclined to 

 orange, which, though it allows the red and green rays to 

 pass freely, is nearly opaque to violet light. 



There is much greater difficulty in procuring light of 

 any other two colours, free from the third ; but the same 

 principle may be traced in a variety of operations in the arts. 



It is a common practice with dyers to give a ground to 

 coarse wool, intended to be dyed blue, by boiling it with 

 cudbear (prepared, I believe, from a species of lichen)* 

 which gives it a crimson red colour : this process is 

 technically called rousing, a corruption probably of rosing, 

 from its giving a crimson or rosy colour. Now, red being 

 the complement of blue, this at first sight appears to be a 

 strange preparation for the purpose of economizing the 

 indigo, which is its object ; for if pure red and pure blue 

 were properly proportioned, the result would be white.f 

 But the red given by cudbear is not pure red, but crimson, 

 a mixture of red and violet ; and the blue given by indigo 

 can only be rendered dark by increasing the proportion of 

 the reflected violet light as compared with the green, its 

 other constituent principle. The crimson ground, then, 

 thus produced, will not reflect green, at least in so large a 

 proportion as it does violet ; and, consequently, the blue 

 colour is rendered dark, or approaching to violet, with a 

 comparatively small quantity of the dye. There is no 

 process in the art of dyeing that without explanation appears 

 more anomalous, or which when explained is more simple 

 and beautiful. 



The operation we have described may be successfully 

 imitated by a combination of coloured glasses. If we take 

 a thin piece of the blue glass of which finger glasses are 

 made, the colour reflected and transmitted by it appears 

 blue; of a light shade ; if we add another piece of the same 

 glass, the colour is darkened, but it requires many thick- 

 nesses to give it the darkest shade ; at which point nearly 



* The Lecanora tartarea. It exists also in Parmelia omphalodes. — Edit. 



t There is a striking instance of this in some experiments made by Mr. Delaval. 

 Purple vegetable colours, reddened by an acid, and then converted to a green by 

 the gradual addition of an alkali, passed through several intermediate stages, one 

 of which, upon this principle, was colourless. 

 VOL. II. I 



