526 M. H. Helmholtz on the Theory of Compound Colours. 



3. Yelloio with green gives a greenish yellow, similar to the 

 tones which lie between these colours in the spectrum. With 

 sky-blue it gives a weak greenisli white ; with indigo-blue, pure 

 white; with violet, a weak flesh-coloured white, which, when 

 violet predominates, passes over into whitish violet, and when 

 yellow predominates, into a whitish yellow. 



4. Green gives with blue, green-blue ; with indigo, a bright 

 blue, which however is much duller and whiter than that of the 

 spectrum ; with violet also it gives a bright blue. 



5. Blue with indigo gives the tones of colour which lie be- 

 tween them ; with violet, a dark blue, which however is less 

 saturated than the indigo of the spectrum. 



6. Indigo with violet gives the intervening tones. 



The most surprising fact, and that which deviates most from 

 the views hitherto entertained, is, that of the colours of the spec- 

 trum there are only two which together give pure white, that is, 

 which are complementary to each other. These are yellow and 

 indigo-blue, two colours from the combination of which it has 

 been hitherto invariably imagined green would result. The 

 yellow which is made use of in this mixture is a very narrow 

 band in the spectrum, lying between the lines D, E, and about 

 three times more distant from E than from D, — a yellow which 

 approximates neither to orange nor to green, and among the 

 pigments is best represented by chromate of lead (chrome-yellow). 

 The blue made use of with this has a greater width, and em- 

 braces the degrees of this colour distinguished by Newton and 

 Fraunhofer as indigo, from about the centre between F and G 

 almost to G. Among pigments, dark ultramarine represents 

 this colour better than the more violet indigo. When the colours 

 to be mixed are obtained from two equally bright spectra of a 

 flint-glass prism, the light used being that ci' the clouds, then 

 the exact centre between the lines E and G is the point which 

 possesses the proper luminous intensity for the production of 

 white. Towards the violet and the line G the blue becomes 

 more feebly luminous, and hence it must here be strengthened 

 in comparison to yellow, in order that white may be obtained. 

 For this reason, in the spectrum of a whitish blue firmament, 

 for example, the white falls near the line G. The brighter blue 

 of the spectrum, which approaches nearer to the line F, gives 

 with pure yellow, by the proper arrangement of their relative 

 intensities, tones which are very similar to white, but which 

 nevertheless arc not without a feeble colouring. The same remark 

 applies to violet when mixed with a greenish yellow. The tint 

 approximates, in most cases, to flesh-colour, or to a bluish and 

 greenish hue; it is, however, sometimes difficult to assign a 

 distinct name to the tint ; but I have never succeeded in obtaining 



