There is, then, a great difference between the _mixing_ of colours, which evokes any tendency to antagonism, and the _adding_ of colours under such conditions that they meet on equal terms. The first process happens, as we have seen, when a ray of light is passed through colours _in succession_; the second, when lights stream simultaneously on to an object. A white screen, being capable of reflecting any colour that falls on to it, will with equal readiness show green, red, violet, or a combination; but a substance that is in white light red, or green, or violet will capture any other colour. So that if for the white screen we substituted a red one, violet or green falling simultaneously, would yield blackness, because red takes _both_ prisoners; if it were violet, green would be captured, and so on. From this follows another phenomenon: that whereas projection of two or more lights may yield white, white cannot result from any mixture of pigments. A person with a whole boxful of paints could not get white were he to mix them in an infinitude of different ways; but with the aid of his lanterns and as many differently coloured glasses the feat is easy enough. Any two colours which meet on equal terms to make white are called _complementary_ colours. Thus yellow (= red + green lights) is complementary of violet. Thus pink (= red + violet lights) is complementary of green. Thus blue (= violet + green lights) is complementary of red. This does not of course apply to mixture of paints, for complementary colours must act together, not in antagonism. If the reader has mastered these preliminary considerations he will have no difficulty in following out the following processes.