that a green sensation is absent, there is no position in any part of the spectrum where there is an absence of light. Had he chosen any other green, the same criticism would have been valid. The diagram as it stands is really a diagram of _colour mixtures_ in terms of three arbitrarily chosen colours, and not of colour _sensations_. It merely indicates what proportions were needed of the three colours, which he took as standards, to match the intermediate spectrum colours. The negative sign in some of the equations--given in the appendix, page 201--may be somewhat puzzling to those who have not made colour matches, but not to those who have actually made experiments. It means that where it is present no match of colour by a mixture of the standard colours is possible; and that it would be only possible if a certain quantity of the colour to which is attached a negative sign were to be abstracted--an impossible condition to fulfil, but one which may often occur in colour-matching experiments. Later you will find that when colours are chosen as standards so that the resulting equations give no negative sign for any colour, we have a criterion as to the colours which give the nearest approach to the true sensations. The next diagram (Fig. 16) of colour sensations is due to Kœnig, who investigated the subject with Von Helmholtz. By a modified method, which perhaps need not be explained in detail here, he produced them, and they must be apparently not far from the actual state of things, supposing this theory be proved to be true. For my own part, I am under the impression that the positions of the colours which most nearly approach the colour sensations might be slightly altered in regard to the green and the blue, for reasons that will subsequently be given when the later experiments of General Festing and myself come to be described. For the immediate purpose of the lecture, the curves are sufficiently accurate, and I will ask you to notice what they tell us. It is presupposed in these diagrams that, if the three colour-perceiving apparatus are equally stimulated, a sensation of white will be produced; and the reverse, of course, is true, in that white will give rise to equal stimulation of the three apparatus. It follows, then, that in the parts of the spectrum where all three curves of sensation are seen to take a part in the production of a colour, such as at the E line, the colour is really due to the extra stimulation of one or two of the apparatus above that required to produce a certain amount of white. _The colour in every part of the spectrum may be represented by not more than two sensations, with a proportion of white._ In the orange and scarlet there are only two sensations excited, without any sensible amount of white, as the amount of violet sensation is extremely small. At the extreme ends of the spectrum only one sensation--the red or the violet--is excited; but