Mr. Galton has further observed that five successive prismatic colours may be combined in such proportions as to produce but one colour, a circumstance which might be of consequence in the art of painting. For if you begin at any part of the circular spectrum above described, and take the first, second, and third colours in the proportions in which they exist in the spectrum; these will compose only the second colour equal in quantity to the first, second, and third; add to these the third, fourth, and fifth in the proportion they exist in the spectrum, and these will produce the fourth colour equal in quantity to the third, fourth, and fifth. Consequently this is precisely the same thing, as mixing the second and fourth colours only; which mixture would only produce the third colour. Therefore if you combine the first, second, fourth, and fifth in the proportions in which they exist in the spectrum, with double the quantity of the third colour, this third colour will be produced. It is probable that many of the unexpected changes in mixing colours on a painter's easle, as well as in more fluid chemical mixtures, may depend on these principles rather than on a new arrangement or combination of their minute particles. Mr. Galton further observes, that white may universally be produced by the combination of one prismatic colour, and a tint intermediate to two others. Which tint may be distinguished by a name compounded of the two colours, to which it is intermediate. Thus white is produced by a mixture of red with blue-green. Of orange with indigo-blue. Of Yellow with violet-indigo. Of green with red-violet. Of blue with Orange-red. Of indigo with yellow-orange. Of violet with green-yellow. Which he further remarks exactly coincides with the theory and facts mentioned by Dr. Robert Darwin of Shrewsbury in his account of ocular spectra; who has shewn that when one of these contrasted colours has been long viewed, a spectrum or appearance of the other becomes visible in the fatigued eye. Philos. Trans. Vol. LXXVI. for the year 1786. These experiments of Mr. Galton might much assist the copper-plate printers of callicoes and papers in colours; as three colours or more might be produced by two copper-plates. Thus suppose some yellow figures were put on by the first plate, and upon some parts of these yellow figures and on other parts of the ground blue was laid on by another copper-plate. The three colours of yellow, blue, and green might be produced; as green leaves with yellow and blue flowers. NOTE III.--COLOURED CLOUDS. _Eve's silken couch with gorgeous tints adorn, Or fire the arrowy throne of rising morn._ CANTO I. l. 119.