Associated with two colours, one sombre, the other luminous, grey will perhaps be better than white, if white produces too strong a contrast of tone: on the other hand, grey will be preferable to black, if that has the inconvenience of increasing too much the proportion of sombre colours. Grey associates more happily than black with orange and violet, green and blue, or green and violet. 288. MIXED GREY. When a ray of solar light (a sunbeam) is passed through a prism of flint glass, and the image or 'prismatic spectrum' received upon a screen of white paper, it is found to consist of numerous rays of different colours, which are conveniently divided into six groups--red, orange, yellow, green, blue, violet. Optically, the union of red, yellow, and blue, in proper proportions, constitutes white light; whether the rays of the three separate colours are mixed, or of one with the other two in combination: the same result ensues when red is mixed with green as if it were mixed with blue and yellow, because green is composed of blue and yellow. Consequently, any primary mixed with a secondary composed of the other two primaries, forms the complement of rays necessary to constitute or make up white light, and vice versâ. There is, however, a very great difference between the results arising from the mixture of the pure coloured rays of the spectrum, and those from material colours or pigments. When, by means of a convex lens, we reunite the coloured rays of the spectrum white light is reproduced; but when we mix coloured materials, blues, yellows, and reds, the compound is never white, but grey or black; even if these coloured pigments are taken in the exact proportions in which their colours exist in the spectrum. Ultramarine, our purest blue, reflects red rays as well as blue rays; aureolin, our purest yellow, reflects blue as well as yellow rays; and carmine reflects yellow as well as red rays. Now whenever the third primary colour is present in any mixture of coloured materials, it tends to form grey, by mixing with a sufficient quantity of the other coloured rays to neutralize it, and the presence of this grey breaks or tarnishes the pure colour. Hence it is that to obtain a pure green, a blue should be taken tinged with yellow rather than with red, and a yellow tinged with blue: if there were chosen either a blue or a yellow tinged with red, this latter colour would go to form some grey in the compound, which would tarnish the green. In like manner, to produce pure orange, neither the red nor the yellow must contain blue; and similarly with pure purple, neither the blue nor red should contain yellow.