[ 209 ] 

 XXX, Intelligence and Miscellaneous Articles. 



ON THE EFFECTS PRODUCED BY MIXING WHITE WITH COLOURED 



LIGHT. BY PROF. 0. N. ROOD, OF COLUMBIA COLLEGE. 

 TT was noticed several years ago that when white light was mixed 

 -*• by the method of rotating disks with light of an ultramarine (arti- 

 ficial) hue, the result was not what one would naturally have ex- 

 pected ; viz. instead of obtaining a lighter or paler tint of violet- 

 blue, the colour inclined decidedly toward violet, passing, when 

 much white was added, into a pale violet hue. Two attempts have 

 been made to account for this curious fact. Briicke supposes that 

 the light which we call white is really to a considerable extent red, 

 and that the mixture of this reddish-white light with the blue 

 causes it to change to violet. Aubert, on the other hand, following 

 a suggestion of Helmholtz, reaches the conclusion that violet is 

 really only a lighter shade of ultramarine-blue. He starts with the 

 assumption that we obtain our idea of blue mixed with white from 

 the sky, which, according to him, is of a greenish-blue colour. "We 

 then apply, as he thinks, this idea to the case of a blue which is 

 not greenish, namely to ultramarine-blue, and are surprised to find 

 that the result is different. 



It will be shown in the present paper that these explanations are 

 hardly correct, since they fail to account for the changes which, 

 according to my experiments, are produced in other colours by an 

 admixture of white. I prepared a set of brilliantly coloured cir- 

 cular disks which represented all the principal colours of the spec- 

 trum and also purple ; these disks were then successively combined 

 in various proportions with a white disk, and the effects of rapid 

 rotation noted, a smaller duplicate coloured disk uncombined with 

 white being used for comparison. Under these circumstances it 

 was found that the addition of white produced the changes indicated 

 in the following table : — 



Vermilion became somewhat purplish. 

 Orange became more red. 

 Yellow became more orange. 

 Greenish yellow was unchanged. 

 Yellowish green became more green. 

 Green became more blue-green 



Cyan-blue became less greenish, more 



bluish. 

 Cobalt-blue became more of a violet 



blue. 

 Ultramarine (artificial) became more 



violet. 



Purple became less red, more violet, 

 Exactly the same effects can be produced by mixing violet with 

 the above-mentioned colours. Let E, Gr, V represent the three 

 angles of Maxwell's colour-triangle, W being the position of white. 

 Now, according to the received theory, as we mix white with differ- 

 ent colours we advance in straight lines 

 from the angles or sides of the triangle 

 toward W; in point of fact, however, I 

 find, as a result of the above-mentioned 

 experiments, that we advance in curves 

 toward W, these curves being similar to 

 those roughly indicated in the figure. 

 The only advance in straight lines is 

 along the line joining violet with its com- 

 plement greenish yellow. The other lines R 



Blue. 

 Cy.B. 



Phil. Mag. S. 5. Yol. 10. No. 61. Sept. 1880. 



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