For some purposes it is advantageous to show experiments before indicating the deductions from them which may lead to a theory. Those described in Chapter IX. will enable us to treat the theory of colour perception from a standpoint of some advantage. How is it that the combination of three colours suffices to form white, or to match any colours we wish, be they spectrum colours to which a little white is added, or the colours of pigments? The most plausible theory that can be advanced is that it is only necessary for the eye to be furnished with a three-colour-perceiving apparatus to give the impression of every colour, and yet this would be somewhat difficult to believe had we not had the experiments narrated in that chapter before us. We should have almost expected some machinery in the eye to exist, which would answer to the rhythmic swing of the rays of every wave-length which together make up white light. But now we have to stand face to face with the results of experiment, and we find that at the most only three colours are necessary to make up white light, and that from these three spectrum colours we can form any others, with the limitation already mentioned, when some simple colours are in question. We must here digress for a moment, and notice the fact that from our experiments we have derived the three primary colours as they are called, viz. red, violet, and green; the definition of a primary colour being that it cannot be formed by the mixture of any other colours. We have ascertained that yellow and blue make white. It is therefore evident that blue, yellow, and red cannot be primary colours, since two of them form white; and we have moreover shown that yellow can be made from green and red; hence it might be fair to assume that the three primary colours are red, green, and blue. But blue, when mixed with a very small percentage of white light, can be made by green and violet. Hence, in the white light formed by the two colours yellow and blue, we have the first made by green and red, and the second by green and violet; hence the three colours which really make the white light are red, green, and violet. The approximate positions of these three colours in the spectrum are those already indicated; though, as we shall presently see, it is highly improbable that any person whose eyes are what are called normal, has ever experienced the fundamental green sensation.