This phenomenon of contrast is one which is most valuable for artistic purposes, for it gives a power of increasing the value of the colour of pigments which is used by the artist almost intuitively. Thus he can heighten the tone of his orange pigment, with which he makes a sunset sky, by placing in juxtaposition with it some bit of blue coloured space. The blue becomes bluer, and the orange more orange, by this artifice. All these artifices--or rather we should say intuitive applications of science--are most necessary when the small range of luminosity of colours with which he has to deal is taken into account. For instance, in a picture of a sun-lighted snow mountain and deep pine forests, the utmost luminosity he can give to the former is that of white paper when seen in the shade, which, in comparison with what he sees, is really a mixture of 90% of black with the light from the snow, so that his range of luminosity is only nine-tenths of that which occurs in nature. It is in adapting this low scale to his picture that true genius of the artist is seen. It might seem that these contrast colours being only a physiological effect, could not be accurately measured, but such is not the case, if a little artifice be employed. If we use the second colour patch apparatus side by side with the first, we can very readily and with very close approximation determine the contrast colours we see. Suppose by the second apparatus we form a colour patch of say red, and place a thin rod in the beam of this ray and of the reflected beam, and about six inches from it form another patch with the first apparatus, using the three slits to make colour mixtures; by first noting the contrast colour, and then approximating in the second patch to what the eye perceives, we can little by little get a fairly exact match to the contrast colour, and can definitely note it. We now give the results of three measures made for the contrast colours which presented themselves to the eye when they were caused by a red ray near the lithium line, another near the E line in the green, and the third near the G line in the violet. To make white light and the contrast colours, the slits had to be of the following apertures-- +-----------------+-------+--------+---------+ | Colour. | Red. | Green. | Violet. | +-----------------+-------+--------+---------+ | White light | 15·7 | 6·5 | 9·8 | | Contrast to Red | 13·5 | 11·8 | 22·5 | | " Green | 15·8 | 5·1 | 4·8 | | " Violet | 15·9 | 7·2 | 4·2 | +-----------------+-------+--------+---------+ Thus to form the contrast to red took 13·5 of red, 11·8 of green, and 22·5 of violet. Now from each of these there can be deducted the amount of white light, which will leave only two colours mixed. Calculating this out we find that the contrasts are--