The final sensation curves for the spectrum colours on the Young theory are still under consideration, and are not definitely fixed, though the observations made have been very numerous. Recently Helmholtz, in the last edition of his “Physiological Optics,” has calculated, from Kœnig’s observations, that no one of the three sensations is singly stimulated by any colour, even at the extreme ends of the spectrum, and he makes the three fundamental sensations vary considerably from those given in these pages. Every colour he states is considerably mixed with white light. The calculations by which he arrived at this conclusion are of a complicated nature, and I think if he had had besides the colour equations of Kœnig, the luminosities and the extinction measures before him, there might have been a modification of his views, for these last give evidence to the contrary. There is a possible modification of the Young theory which would account for a good many of the phenomena that are unaccounted for by it in its present form, though it may raise new difficulties in the minds of some. Let us suppose that each of the three sensations were compounded of fundamental _light and of colour_ in fixed and definite proportions, and not in the same proportion in each; and further that the apparatus in the eye which was responsible for each sensation had two functions, one of which was to respond to the fundamental light sensation and the other to the colour. One essential difference between this modification of the Young theory and that of Hering is that, whilst in the latter the white sensation is a sensation _distinct from the colour sensations_, in the former it is a _definite part_ of them. The fact that the sensation of colour is lost before the sensation of light is one of the greatest significance, and any theory to be accepted must offer a reasonable explanation of it. If the modification suggested be made, it accounts for the existence of this residuum of light equally as well as Hering’s theory, and without its drawback. It is not hard to imagine the apparatus which gives rise to two sensations, on the assumption of different kinds of atomic motion, induced by the ether motion, or at least three kinds are possible. When extinction of _colour_ is made, the ether vibrations would have sufficient energy to induce but one kind of motion; and when all _light_ was extinguished from the same ray, they would not be capable of inducing any sensible motion whatever. In the case of Miss W., who saw all colours as white, it might be that disease had entirely prevented the first kind of motion in all three sensations, and that in P. and Q. the red and green sensations were absent or paralysed in their entirety, whilst the blue sensation was left in full operation. In B. C. the blue and red sensations would be similarly absent, leaving the green sensation unchanged. The coincidence of their persistency and luminosity curves would then indicate that