BY JOHN THOMSON, M.B. 31 



practically to solve the colour problem, for it suited every kind of 

 worker in every kind of pigment. The secondaries were easily 

 obtainec. Red and yellow gave orange ; yellow and blue 

 produced green, while blue and red made the violets ; the 

 tertiaries — russet, buff, citron, sage, slate, and plum were 

 readily accounted for, even if it has to be admitted that they 

 were but primaries or secondaries dulled with grey. 



Scientific enquiry demonstrated that pigments, the finest 

 and the best of them in their perfection did not compare in 

 purity with the simple colours of the spectrum, and that a 

 blending of any of these gave different results from the mixing 

 of similarly named paints or dyes. 



Every one knows that in painting yellow and blue make 

 green, but what every one does not know is that the pure yellow 

 and blue of the spectrum when blended, instead of green, give 

 a pure white, and that green cannot be formed by a mixture 

 of any two colours, and that yellow can, for a mixture of red 

 and green gives yellow, therefore yellow is not a primary while 

 green is. 



Dr. Thomas Young (1773-1829), nearly a century ago, 

 founded his theory " that white light is composed of a mix- 

 ture of three colours only — red, green, and violet." Von Helm- 

 holtz (1821-1894) more fully developed this theory, which is 

 now known as the " Young-Helmholtz," and is generally accepted 

 by physicists as satisfactory. 



Professor Clark-Maxwell (1831-1879) modified Young's by 

 difi'ering as to the hues of the primaries. His colours 

 were — pure red or scarlet, pure green, and pure blue. 

 A solution of sulphocyanide of iron fairly well matches the first ; 

 chloride of copper, the second ; and ammoniacal sulphate of 

 copper, the third ; and with troughs of these and three lanterns, 

 satisfactory experiments with this theory may be made. 



Hering of Vienna, in 1878, investigated the subject and 

 proposed a theory of six primaries : — White, black, red, yellow, 

 green and blue ; these were arranged in complementary pairs — 

 white and black, red and green, yellow and blue. This theory 

 was founded on "fundamental or native sensations," on a purely 

 physiological — not physical— basis, and was supposed to repre- 

 sent common experience. 



The Eye. 



The photographic camera and the human eye are both 

 optical picture producers and have many things in common ; 



