Five of Maxwells Papers | Page 2

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. In all these cases blue and yellow do not make green. I have
also made experiments on the mixture of coloured powders. Those
which I used principally were "mineral blue" (from copper) and
"chrome-yellow." Other blue and yellow pigments gave curious results,
but it was more difficult to make the mixtures, and the greens were less
uniform in tint. The mixtures of these colours were made by weight,
and were painted on discs of paper, which were afterwards treated in
the manner described in my paper "On Colour as perceived by the
Eye," in the _Transactions of the Royal Society of Edinburgh_, Vol.
XXI. Part 2. The visible effect of the colour is estimated in terms of the
standard-coloured papers:--vermilion (V), ultramarine (U), and
emerald-green (E). The accuracy of the results, and their significance,
can be best understood by referring to the paper before mentioned. I
shall denote mineral blue by B, and chrome-yellow by Y; and B3 Y5
means a mixture of three parts blue and five parts yellow.

Given Colour. Standard Colours. Coefficient V. U. E. of brightness.
B8 , 100 = 2 36 7 ............ 45 B7 Y1, 100 = 1 18 17 ............ 37 B6 Y2,
100 = 4 11 34 ............ 49 B5 Y3, 100 = 9 5 40 ............ 54 B4 Y4, 100 =
15 1 40 ............ 56 B3 Y5, 100 = 22 - 2 44 ............ 64 B2 Y6, 100 = 35
-10 51 ............ 76 B1 Y7, 100 = 64 -19 64 ............ 109 Y8, 100 = 180
-27 124 ............ 277
The columns V, U, E give the proportions of the standard colours
which are equivalent to 100 of the given colour; and the sum of V, U, E
gives a coefficient, which gives a general idea of the brightness. It will
be seen that the first admixture of yellow diminishes the brightness of
the blue. The negative values of U indicate that a mixture of V, U, and
E cannot be made equivalent to the given colour. The experiments from
which these results were taken had the negative values transferred to
the other side of the equation. They were all made by means of the
colour-top, and were verified by repetition at different times. It may be
necessary to remark, in conclusion, with reference to the mode of
registering visible colours in terms of three arbitrary standard colours,
that it proceeds upon that theory of three primary elements in the
sensation of colour, which treats the investigation of the laws of visible
colour as a branch of human physiology, incapable of being deduced
from the laws of light itself, as set forth in physical optics. It takes
advantage of the methods of optics to study vision itself; and its appeal
is not to physical principles, but to our consciousness of our own
sensations.
*** On an Instrument to illustrate Poinsot's Theory of Rotation.
James Clerk Maxwell
[From the Report of the British Association, 1856.]
In studying the rotation of a solid body according to Poinsot's method,
we have to consider the successive positions of the instantaneous axis
of rotation with reference both to directions fixed in space and axes
assumed in the moving body. The paths traced out by the pole of this
axis on the invariable plane and on the _central ellipsoid_ form

interesting subjects of mathematical investigation. But when we
attempt to follow with