Page:Scientific Memoirs, Vol. 1 (1837).djvu/128

116 pure: they are all composed of several others. Hence arises a law of brightness different from that of the prismatic colours. The clearest on our scale are, The most obscure tints are Nos. 10, 11, and 12, in which the violet and blue predominate.

Depth.

Depth, or intensity, and brightness are very different qualities. No one indeed confounds the intensity of a fine red with the brightness of a fine yellow. In the scale of the latter quality the white occupies the first place. A bright tint may be considered as a mixture, in which there is a little colour with a great quantity of white light; and, vice versâ, a strong or deep colour, as a mixture of much colour with a little white light. Painters therefore when they want to give brightness to their colours add white, but when they want to increase their intensity they add a different colour.

The most intense colours of the scale are the lakes, especially No. 28 and No. 29. The feeblest are the azures No. 16 and No. 17, the blonds No. 1 and No. 2, and the yellow No. 18.

Some colours strengthen each other; some have no such effect. Thus, for example, the red of the spectrum combined with the violet forms a very beautiful lake, which is a much more vivid red than that of the prism. The same red combined with the green forms a mixture which possesses more intensity. The tints of the scale include all the prismatic colours, and their strength depends exactly on the proportion of the elements which enter into their composition. The lakes abound in red and violet, which are the two colours that give most depth to each other, as if one were the octave of the other. The sky-colours are too feeble, because with the exception of the blue, which they contain in a quantity rather excessive, the colours which enter into their composition will, when mixed, produce only white.

It is not strictly true that the intensity is in the inverse ratio of the brightness, because the more obscure tints Nos. 10, 11 and 12 are less intense than the lakes Nos. 28 and 29. Nevertheless there is a manifest relation between the two qualities; for it is certain that the feeblest colours are among those of the brightest class, and the most intense among those of the most obscure.

Thin plates according to their different degrees of tenuity reflect different colours; either these reflected colours are such as mutually to strengthen each other, so that there results from them a strong tint; or they do not strengthen each other, and the result of this is a white which predominates in the tint. Thus, the cause which generally renders