80 
PEOrESSOE J. CLEEK MAXWELL OX 
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^ 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)4-32(88) + 31(68)=W; (4.) 
and the value of D, as deduced from these observers, would have been 
23(104)-l-7(88)-l-l(68)=D, (5.) 
in which the defective sensation is much nearer to the red of the spectrum. It is pro- 
bably 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. 
From other observations by Mr. Simpson the following results have been deduced ; — 
Table a. 
(99-2 + )=: 
(88). 
33-7 
(68). 
1-9 
(88). 
(68_) 
31-3(96)= 
33-7 
2-1 
100(96)= 
108 
7 
28 (92)= 
33-7 
1-4 
100(92)= 
120 
5 
33-7(88)= 
33-7 
0 
100(88)= 
100 
0 
54-7(84)= 
33-7 
6-1 
100(84)= 
61 
11 
71 (82)= 
33-7 
15-1 
100(82)= 
47 
21 
99 (80)= 
33-7 
33-1 
100(80) = 
34 
33 
70 (78)= 
15-7 
33-1 
100(78)= 
22 
47 
56 (76)= 
5-7 
33-1 
100(76)= 
10 
59 
36 (72)=- 
0-3 
33-1 
100(72)= 
- 1 
92 
33-1(68)= 
0 
33-1 
100(68)= 
0 
100 
40 (64)= 
0-2 
33-1 
100(64)= 
0 
83 
55-5(60)= 
1-7 
33-1 
100(60)= 
o 
o 
60 
(57-) =- 
0-3 
33-1 
i 
In the Table on the left side (99'2-l-) 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. 
The values of the standard colours in different parts of the spectrum are given on the 
* Philosophical Tz’aDsactious, 1859, Part 1. p, 329. 
