146 EXPERIMENTS ON COLOUR, AB PERCEIVED BY THE EYE. 



to form gray, Professor Forbes endeavoured to form a neutral tint, by the 

 combination of three colours only. For this purpose, he combined the three 

 so-called primary colours, red, blue, and yellow, but the resulting tint could 

 not be rendered neutral by any combination of these colours; and the reason 

 was found to be, that blue and yellow do not make green, but a pinkish tint, 

 when neither prevails in the combination. It was plain, that no addition of 

 red to this, could produce a neutral tint. 



This result of mixing blue and yellow was, I believe, not previously known. 

 It directly contradicted the received theory of colours, and seemed to be at 

 variance with the fact, that the same blue and yellow paint, when ground 

 together, do make green. Several experiments were proposed by Professor Forbes, 

 in order to eliminate the effect of motion, but he was not then able to under- 

 take them. One of these consisted in viewing alternate stripes of blue and 

 yellow, with a telescope out of focus. I have tried this, and find the resultant 

 tint pink as before*. I also found that the beams of light coloured by trans- 

 mission through blue and yellow glasses appeared pink, when mixed on a screen, 

 while a beam of light, after passing through both glasses, appeared green. By 

 the help of the theory of absorption, given by Herschelt, I made out the 

 complete explanation of this phenomenon. Those of pigments were, I think, first 

 explained by Helmholtz in the manner above referred toj. 



It may still be asked, whether the effect of successive presentation to the 

 eye is identical with that of simultaneous presentation, for if there is any action 

 of the one kind of light on the other, it can take place only in the case of 

 simultaneous presentation. An experiment tending to settle this point is recorded 

 by Newton (Book I. Part n., Exp. 10). He used a comb with large teeth to 

 intercept various rays of the spectrum. When it was moved slowly, the various 

 colours could be perceived, but when the speed was increased the result was 

 jierfect whiteness. For another form of this experiment, see Newton's Sixth 

 Letter to Oldenburg (Horsley's Edition, Vol. IV., page 335). 



In order more fully to satisfy myself on this subject, I took a disc in 

 which were cut a number of slits, so as to divide it into spokes. In a plane, 

 nearly passing through the axis of this disc, I placed a blue glass, so that one 



* See however Encyc. Metropolilana, Art. "Light," section 502. t Ib. sect. 516. 



t I have lately seen a passage in Moigno's Cosmos, stating that M. Plateau, in 1819, had obtain^! 

 gray by whirling together gamboge and Prussian blue. Corresjxmdance Jfatft. et Phys. de M. Quetelet, 

 Vol. v., p. 221. 



