292 MR J. CLERK MAXWELL ON COLOUR, 



of this phenomenon. Those of pigments were, I think, first explained by Helm- 

 holtz in the manner above referred to.* 



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 II., 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 perfect whiteness. 

 For another form of this experiment, see Newton's Sixth Letter to Oldenbueg 

 (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 half of the 

 disc might be seen by transmitted light — blue, and the other by reflected light — 

 white. In the course of the reflected light I placed a yellow glass, and in this 

 way I had two nearly coincident images of the slits, one yellow and one blue. 

 By turning the disc slowly, I observed that in some parts the yellow slits and 

 the blue slits appeared to pass over the field alternately, while in others they 

 appeared superimposed, so as to produce alternately their mixture, which was 

 pale pink, and complete darkness. As long as the disc moved slowly I could 

 perceive this, but when the speed became great, the whole field appeared uni- 

 formly coloured pink, so that those parts in which the colours were seen succes- 

 sively were indistinguishable from those in which they were presented together 

 to the eye. 



Another form in which the experiment may be tried requires only the colour- 

 top above described. The disc should be covered with alternate sectors of any 

 two colours, say red and green, disposed alternately in four quadrants. By 

 placing a piece of glass above the top, in the plane of the axis, we make the image 

 of one half seen by reflection coincide with that of the other seen by transmission. 

 It will then be seen that, in the diameters of the top which are parallel and per- 

 pendicular to the plane of reflection, the transmitted green coincides with the 

 reflected green, and the transmitted red with the reflected red, so that the result 

 is always either pure red or pure green. But in the diameters intermediate to 

 these, the transmitted red coincides with the reflected green, and vice versa, so that 

 the pure colours are never seen, but only their mixtures. As long as the top is 

 spun slowly, these parts of the disc will appear more steady in colour than those 

 in which the greatest alternations take place ; but when the speed is sufficiently 



* I have lately seen a passage in Moigno's Cosmos, stating that M. Plateau, in 1819, had ob- 

 tained gray by whirling together gamboge and Prussian blue. — Correspondence Math, et Phys., de 



M. QlJETELET, Vol. V., p. 221. 



