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 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 circumstances of motion, by means of a top of the same kind as that used by Mr Elliot, to illustrate precession*.  The body of the instrument is a hollow cone of wood, rising from a ring, 7 inches in diameter and 1 inch thick.  An iron axis, 8 inches long, screws into the vertex