The clearest explanation of the scientific theory of colouring is to be found in the treatise written by Chevreul. First published in France in 1838, it met with great success, and was translated into English in 1854 by Charles Martel. Chevreul remains the standard authority, although he has been followed by Helmholtz, Church, Rood, and others. Given the necessary competence for accuracy in draughtsmanship, and considerable practice in the manipulation of colour, the art-student may take the field, and not before; for Impressionist painting demands the highest artistic capability. Firstly, he will discover that Impressionists worship light, using the trees, rocks, rivers, &c. of landscape, as so many vehicles for the conveyance of luminous impressions to the eye. This quality of atmosphere distinguishes Impressionist pictures from all others; here will be found what Brownell, Chevreul, MacColl, and Mauclair, have to say upon the subject. Secondly, the art-student will perceive the vital necessity of correct values within a general tone, a subject also enlarged upon by the above writers. Thirdly, some reference is given to the modern study of shadows and reflections, with regard to their influence and treatment. The following lines, extracted from “The French Impressionists,” by Camille Mauclair, sum up definitely the Impressionist Idea. “In nature no colour exists by itself. The colouring of the objects is a pure illusion: the only creative source of colour is the sunlight which envelops all things, and reveals them, according to the hours, with infinite modifications.... Only artificially can we distinguish between outline and colour; in nature the distinction does not exist.... A value is the degree of dark or light intensity, which permits our eyes to comprehend that one object is further or nearer than another.... The values are the only means that remain for expressing depth on a flat surface. Colour is therefore the procreatrix of design. Colour being simply the irradiation of light, it follows that all colour is composed of the same elements as sunlight, namely the seven tones of the spectrum.... The colours vary with the intensity of light. There is no colour peculiar to any object, but only more or less rapid vibration of light upon its surface. The speed depends, as is demonstrated by optics, on the degree of the inclination of the rays which, according to their vertical or oblique direction, give different light and colour.... What has to be studied therefore in these objects, if one wishes to recall their colour to the beholder of a picture, is the composition of the atmosphere which separates them from the eye. This atmosphere is the real subject of the picture, and whatever is represented upon it only exists through its medium. A second consequence of this analysis of light is, that shadow