Assuming that persons of ordinary vision see three simple colours, red, yellow, and blue, and that all the rest of the colours are mixtures of these with each other and with white light, let us try to picture to ourselves what must be the visual condition of a person who is unable to recognize certain rays; and as it appears that there is but one kind of colour-blindness known, we will assume that the person is unable to recognize those rays of white light which consist of pure red and nothing else. In other words, let us investigate the sensations of a person blind so far only as pure red is concerned. All visible objects either reflect the same kind of light as that which falls on them, absorbing part and reflecting the rest, or else they absorb more of some colour rays than others, and reflect only a negative tint, made up of a mixture of all the colour-rays not absorbed. To a colour-blind person, the mixed light, as it proceeds from the sun, is probably white, as seen by those having perfect vision; for, as we have explained already, positive blue and yellow (the colour rays when red is excluded) do not make green, and the absence of the red ray is likely to produce only a slight darkening effect. So far, then, there is no difference. But how must it be with regard to colour. Bearing in mind what has been said above, it is evident that in withdrawing the red rays from the spectrum, we affect all the colours. The orange is no longer red and yellow, but darkened yellow; the yellow is purer, the green is quite distinct, the blue purer, and the indigo and violet no longer red and blue, but blue mingled with more or less of darkness, the violet being the darkest, as containing least blue in proportion to red, while the red part itself, though not seen as a colour, is not absolutely black, inasmuch as its part of the spectrum is faintly coloured with the few mixed rays of blue and yellow and white that escape from their proper place. The red then ought to be seen as a gray neutral tint, the orange a dingy yellow, the indigo a dirty indigo, and the violet a sickly, disagreeable tint of pale blue, darkened considerably with black and gray.