“In making experiments, the instrument was placed on a table in a room moderately lighted, with the end A B turned towards a large board covered with white paper, and placed in the open air, so as to be uniformly illuminated by the sun. In this way the three slits and the mirror M were all illuminated with white light of the same intensity, and all were affected in the same ratio by any change of illumination; so that if the two halves of the field were rendered equal when the sun was under a cloud, they were found nearly correct when the sun again appeared. No experiments, however, were considered good unless the sun remained uniformly bright during the whole series of experiments. “After each set of experiments light was admitted at E, and the position of the fixed lines D and F of the spectrum was read off on the scale at A B. It was found that after the instrument had been in use some time these positions were invariable, showing that the eye-hole, the prisms, and the scale might be considered as rigidly connected.” [Illustration: FIG. 15.] With this instrument he made mixtures of three colours, to match with white. By shifting the slits into various positions and taking as his three standard colours a red near the C line, a green near E, and a blue between F and G (see frontispiece), he obtained a variety of matches, from which he formed equations. After eliminating, or rather reducing the errors to the most probable value by the method of least squares, he got from his matches with white a table of colour values in terms of the three standard colours, from which the diagram of the spectrum (Fig. 15) was made. (The heights of the dotted curves are derived from the widths of the slits, and the continuous curve is the sum of these heights.) Now what appears to be a properly chosen colour does not necessarily stimulate only one sensation. Indeed the probabilities are against it, except in the extreme red and extreme violet. If colours intermediate to the standard colours be matched by a mixture of the latter, we do not arrive at any solution of the amount of stimulation of each sensation, since the chosen standard colours themselves may be due to a stimulation of all three sensations. As a matter of fact, Clerk Maxwell chose colours which do not best represent the colour sensations. The red is too near the yellow, as is also the green. The blue should also be nearer the violet end of the spectrum than the position which he chose for it. We may take it, then, that except as a first approximation, Clerk Maxwell’s diagrams need not be seriously taken into account. The diagram itself shows that the colour _sensations_ are not represented by the colours he chose. Supposing any one in whom the sensation of green is absent were examining the spectrum, there would, according to the diagram, be no light visible at the green at E. Anticipating for a moment what we shall deal with in detail shortly, it may be stated that in cases where it is proved