Prof. J. D. Forbes of Edinburgh made similar experiments in 1849, with
the same result. Prof. Helmholtz of Konigsberg, to whom we owe the
most complete investigation on visible colour, has given the true
explanation of this phenomenon. The result of mixing two coloured
powders is not by any means the same as mixing the beams of light
which flow from each separately. In the latter case we receive all
the light which comes either from the one powder or the other. In the
former, much of the light coming from one powder falls on particles of
the other, and we receive only that portion which has escaped
absorption by one or other. Thus the light coming from a mixture of
blue and yellow powder, consists partly of light coming directly from
blue particles or yellow particles, and partly of light acted on by
both blue and yellow particles. This latter light is green, since the
blue stops the red, yellow, and orange, and the yellow stops the blue
and violet. I have made experiments on the mixture of blue and yellow
light--by rapid rotation, by combined reflexion and transmission, by
viewing them out of focus, in stripes, at a great distance, by
throwing the colours of the spectrum on a screen, and by receiving
them into the eye directly; and I have arranged a portable apparatus
by which any one may see the result of this or any other mixture of
the colours of the spectrum.
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