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.]