From six observations of my own, made at the same time, I have deduced the “trichromic” equation-- 22·6 (104) + 26 (88) + 37·4 (68) = W (2) If we suppose that the light which reached the organ of vision was the same in both cases, we may combine these equations by subtraction, and so find 22·6 (104) - 7·7 (88) + 4·3 (68) = D (3) where D is that colour, the absence of the sensation of which constitutes the defect of the dichromic eye. The sensation which I have in addition to those of the dichromic eye is therefore similar to the full red (104), but different from it in that the red (104) has 7·7 of green (88) in it which must be removed, and 4·3 of blue (68) substituted. This agrees pretty well with the colour which Mr. Pole[A] describes as neutral to him, though crimson to others. It must be remembered, however, that different persons of ordinary vision require different proportions of the standard colours, probably owing to differences in the absorptive powers of the media of the eye, and that the above equation (2), if observed by K., would have been 23 (104) + 32 (88) + 31 (68) = W (4) and the value of D, as deduced from these observers, would have been 23 (104) - 1·7 (88) - 1·1 (68) = D (5) in which the defective sensation is much nearer to the red of the spectrum. It is probably a colour to which the extreme red of the spectrum tends, and which differs from the extreme red only in not containing that small proportion of “yellow” light which renders it visible to the colour blind. [A] Philosophical Transactions, 1859, Part I., p. 329. From other observations by Mr. Simpson the following results have been deduced:-- TABLE A. (88) (68) | (88) (68) (99·2 +) = 33·7 1·9 | 100 (96) = 108 7 31·3 (96) = 33·7 2·1 | 100 (92) = 120 5 28 (92) = 33·7 1·4 | 100 (88) = 100 0 33·7 (88) = 33·7 0 | 100 (84) = 61 11 54·7 (84) = 33·7 6·1 | 100 (82) = 47 21 71 (82) = 33·7 15·1 | 100 (80) = 34 33 99 (80) = 33·7 33·1 | 100 (78) = 22 47 70 (78) = 15·7 33·1 | 100 (76) = 10 59 56 (76) = 5·7 33·1 | 100 (72) = 1 92 36 (72) = 0·3 33·1 | 100 (68) = 0 100 33·1 (68) = 0 33·1 | 100 (64) = 0 83 40 (64) = 0·2 33·1 | 100 (60) = 3 60 55·5 (60) = 1·7 33·1 | (57 -) = 0·3 33·1 | In the table on the left side (99·2 +) means the whole of the spectrum beyond (99·2) on the scale, and (57 -) means the whole beyond (57) on the scale. The position of the fixed lines with reference to the scale was as follows:-- A, 116; a, 112; B, 110; C, 106; D, 98·3; E, 88; F, 79; G, 61; H, 44.