260 Prof. Grassmami oa the Theory uf Compound Colours. 



any of his series of researches, as, for instance, that iipou the 

 mixture of red \Aith the other colours, we shall always readily 

 obtain the coni})lemcntary colour. According to him, red gives 

 with orange, yellow and green, the intermediate tones of colour 

 which lie in this series, and therefore, according to our denomi- 

 nation, on the positive side of red. Tims, for example, according 

 to Hclmholtz, red mixed with green gives a pale yellow, which, 

 when red predominates, passes over through orange into red ; 

 and when green prevails, passes through yellowish- green to green. 

 In the same manner, red, with violet, indigo, and sky-blue, fur- 

 nishes the intermediate tones of colour, which, according to us, 

 lie on the negative side of red. According to Helmholtz, red 

 mixed with azure-blue gives a whitish violet, which, when red 

 predominates, passes into rose-colour and carmine. Thus, from 

 the propositions proved above, the complementary colour of red 

 must lie between green and azure-blue, and must therefore be a 

 tint of bluish-green. Now Helmholtz says that the mixture 

 of red with the greenish- blue tones produces a flesh-colour, but 

 nothing is said as to how this flesh-colour passes over into bluish- 

 green when this tint is in excess, although this must be the case. 

 There is consequently a deficiency here. Moi-eover flesh-colour 

 is nothing but a red mixed with much white, and it has no other 

 conceivable transition to bluish-green, except by the gradual 

 weakening of the red until it entirely disappears before the white, 

 and the gradual production of bluish-green from this white (or 

 gray) ; in short, the normal transition through colourless light 

 takes place in this case. The same applies to the other series of 

 experiments. The table of the complementary colours derived 

 from them would be as follows : — 



Yellow, Yellowish-green, Green, Bluish-green, Azure, Indigo, 

 Indigo, Violet, Purple, Red, Orange, Yellow, 



in which the complementary colours stand one above another. 



I have hitherto endeavoured to make as few assumptions as 

 possible suffice. I will now, in order to introduce the main 

 principle of compound colours, add a third assumption to the 

 two preceding ones, namely, — 



"That two colours, each of which has a constant tint and a 

 constant intensity of the intermixed white, also give constant 

 mixed colours, no matter of what homogeneous colours they may 

 be composed." 



This proposition also appears to be sufficiently proved by the 

 preceding observations ; for that coloured powders, when mixed, 

 furnish results differing from those obtained when the light 

 proceeding from them only is mixed, can form no objection, 

 especially as Helmholtz has disclosed the cause of this difl"erence. 



