THE THEORY OF COLOR. 59 



that the three primary colors were red, green, and 

 violet ; but his results attracted little notice, and the 

 general view used to be that there were seven principal 

 colors red, orange, yellow, green, blue, indigo, and 

 violet ; four of which namely orange, green, indigo, 

 and violet were considered to arise from mixtures of 

 the other three. Red, yellow, and blue were therefore 

 called the primary colors, and it was supposed that in 

 order to produce white light these three colors must 

 always be present. 



Helmholtz, however, again showed, in 1852, that a 

 color to our unaided eyes identical with white, was 

 produced by combining yellow with indigo. At that 

 time yellow was considered to be a simple color, and 

 this, therefore, was regarded as an exception to the 

 general rule, that a combination of three simple colors 

 is required to produce white. Again, it was, and indeed 

 still is, the general impression that a combination of 

 blue and yellow makes green. This, however, is 

 entirely a mistake. Of course we all know that yellow 

 paint and blue paint make green paint : but this results 

 from absorption of light by the semi-transparent solid 

 particles of the pigments, and is not a mere mixture of 

 the colors proceeding unaltered from the yellow and 

 the blue particles ; moreover, as can easily be shown by 

 two sheets of colored paper and a piece of window 

 glass, blue and yellow light, when combined, do not 

 give a trace of green, but if pure would produce the 

 effect of white. Green, therefore, is after all not pro- 

 duced by a mixture of blue and yellow. On the other 

 hand, Clerk-Maxwell proved in 1860 that yellow could 

 be produced by a mixture of red and green, which put 



