292 MR J. CLERK MAXWELL ON COLOUR, 
of this phenomenon. Those of pigments were, I think, first explained by Heum- 
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 Newron 
(Book I. Part IL, 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 Newron’s Sixth Letter to OLDENBURG 
(Horstey’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. Inthe 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 ferm 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. 
Jt 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 Mo1qno’s Cosmos, stating that M. Prarerav, in 1819, had ob- 
tained gray by whirling together gamboge and Prussian blue.—Correspondence Math. et Phys., de 
M. QueteLet, vol. v., p. 221. 
