The slits may be set at these apertures and a colour patch thrown on the screen, and we shall find it of a delicate pink. The truth of this can be seen by using a double-image prism to view the pigmented surface, illuminated by the same white light as that in which it was measured, and the colour patch on the screen by its side. The two colours may be caused to overlap, when it will be seen that white is produced. Another example was an orange pigment, and this we will work out in the form of colour equation. The same mixture gave white, viz.: 165 R + 60 G + 100 V = W 165 R + 42 G = O ∴ the complementary colour, which is W - O = 18 G + 100 V, or a dark-blue colour. In this case there was apparently no white light reflected from the orange. It was slightly glossy, and as polarized light was used for the reflected beam, it was probably somewhat quenched; but what is more probable is that the green contains some violet as well as red, for the reasons given in chapter XI. The reason we have been particular in showing to what extent complementary colours must be diluted with white to the same proportion that the colour itself is diluted, will be apparent if considered for a moment. A deep brown is in reality orange, much degraded in tone, and can be produced as a colour patch on the screen if a bright orange pigment be placed in the reflected beam of the colour patch, and the light nearly shut off by the rotating sectors. Now the same complementary colour will be found for both, but if we were to use the bright complementary colour which we obtained with the orange for the brown, and endeavoured to obtain a white with it by means of the double-image prism we should fail, as the complementary colour would predominate. Complementary colours can always be formed by a mixture of only two rays, and although the overlapping images may form white, yet when the two are placed side by side, it often will be found that the complementary, unless diluted with white, is evidently too dark to be satisfactory, but the luminosity may be increased by adding white to it, as any amount of white may be added to the mixture of the two rays which form the complementary, and of course white will still be formed with the original colour. It is thus quite feasible to give the complementary the same luminosity as the latter by adding white light to it. Like the colour itself, the complementary colour can always be expressed either by a single ray of the spectrum, or by white light from which a single ray is deducted. (See chapter XIII.) CHAPTER XV. Persistence of Images on the Retina--The Use of Coloured Discs. Fig. 39.--Disc to cause alternate opening and closing of two Slits.