^{page 442 note *} I take this term from Donders; the defect has sometimes been called “Monochromic” vision, but this is inapplicable if we assume that colour is something distinct from the sensation of ordinary light, as we do when we use the now well-established term “Dichromic.” On this view it is difficult to understand how there can be any monochromic vision, as, if the light is at all varied by refraction, it must be broken up into at least two colours. No doubt white has often to be treated as a colour-sensation; but the popular use of the term “coloured,” as distinguished from the simple sensation of light, justifies the above nomenclature.

^{page 443 note *} Sir John Herschel, being interested in Dalton's description of his vision of colours, had sent him some “Optical Queries,” accompanied by test-glasses and by samples of coloured silks, to which Dalton sent replies, as mentioned in the letter, which was in fact Herschel's judgment upon them. At a later time Sir John put these data in my possession, and I was enabled thereby to discover the exact particulars of the nature of Dalton's vision.—See Phil. Mag., July 1892.

^{page 445 note *} The exact matching of these three colour-impressions will vary slightly in different dichromic eyes, for reasons hereafter given; with some patients the external squares in the figure will appear slightly coloured, and varying in intensity. Indeed the appearances may vary slightly, even in the same individual, with variations of the illumination.

^{page 446 note *} See further explanations on this point in the Phil Mag., July 1892.

^{page 446 note †} These particulars were given in my original paper of 1856 (sent to the Royal Society of London), but were omitted in printing. I made a postscript in 1859 that I had discovered that the colour-top equations varied to some extent, even in the same individual, at different times. This was afterwards explained by the variations of the light which illumined them.

^{page 447 note *} Hering has noticed this, and has offered an explanation of it. See Phil. Mag., August 1893.

^{page 448 note *} That is, subject to exceptions caused by personal variations,—which, according to recent researches, may sometimes be considerable.

^{page 450 note *} See, however, further explanations of Holmgren's general views on pp. 453, 454, 455, and 461.

^{page 457 note *} See abstract of his views given by myself in the Phil. Mag. for November 1892.

^{page 466 note *} I have published an account of Hering's General Colour-Theory in Nature for 1879; and an abstract of his subsequent Colour-Blindness Essay in the Phil. Mag. for August 1893. See also Note on page 477 of this article.

^{page 467 note *} In Donders's writings he laid some stress on my case; and I hope I may be pardoned for quoting the following passage from one of his letters to me, dated 15th March 1881:—“was very happy to mention your case as the model case; I mean, your description as the model description, even for the present time. It is indeed admirable, and I understand now perfectly why it was appreciated so highly by Sir John Herschel.”

^{page 468 note *} See my remarks on the subject in Nature, 1878.

^{page 478 note *} This appears to have been an oversight, as the idea of tetrachromatism was not, to my knowledge, included in Helmholtz's suggestions.