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 our eye the motion of a rotating
body, we find it difficult to determine through what point of the
_body_ the instantaneous axis passes at any time,--and to determine its
path must be still more difficult. I have endeavoured to render
visible the path of the instantaneous axis, and to vary the