The third conclusion resulting from this: the colours in the shadow are modified by _refraction_. That means, _f.i._ in a picture representing an interior, the source of light (window) may not be indicated: the light circling round the picture will then be composed of the _reflections_ of rays whose source is invisible, and all the objects, acting as mirrors for these reflections, will consequently influence each other. Their colours will affect each other, even if the surfaces be dull. A red vase placed upon a blue carpet will lead to a very subtle, but mathematically exact, interchange between this blue and this red, and this exchange of luminous waves will create between the two colours a tone of reflections composed of both. These composite reflections will form a scale of tones complementary of the two principal colours. The science of optics can work out these complementary colours with mathematical exactness. If _f.i._ a head receives the orange rays of daylight from one side and the bluish light of an interior from the other, green reflections will necessarily appear on the nose and in the middle region of the face. The painter Besnard, who has specially devoted himself to this minute study of complementary colours, has given us some famous examples of it. The last consequence of these propositions is that the blending of the spectral tones is accomplished by a _parallel_ and _distinct_ projection of the colours. They are artificially reunited on the crystalline: a lens interposed between the light and the eye, and opposing the crystalline, which is a living lens, dissociates again these united rays, and shows us again the seven distinct colours of the atmosphere. It is no less artificial if a painter mixes upon his palette different colours to compose a tone; it is again artificial that paints have been invented which represent some of the combinations of the spectrum, just to save the artist the trouble of constantly mixing the seven solar tones. Such mixtures are false, and they have the disadvantage of creating heavy tonalities, since the coarse mixture of powders and oils cannot accomplish the action of light which reunites the luminous waves into an intense white of unimpaired transparency. The colours mixed on the palette compose a dirty grey. What, then, is the painter to do, who is anxious to approach, as near as our poor human means will allow, that divine fairyland of nature? Here we touch upon the very foundations of Impressionism. The painter will have to paint with only the seven colours of the spectrum, and discard all the others: that is what Claude Monet has done boldly, adding to them only white and black. He will, furthermore, instead of composing mixtures on his palette, place upon his canvas touches of none but the seven colours _juxtaposed_, and leave the individual rays of each of these colours to blend at a certain distance, so as to act like sunlight itself upon the eye of the beholder. [Illustration: DEGAS