ON COLOUR PHOTOMETRY. 
551 
Table VI.—M.’s Luminosity Curve compared with the Normal. 
I. 
II. 
III. 
IV. 
V. 
VI. 
VII. 
Scale 
number. 
Wave¬ 
length. 
Mean 
reading. 
Mean 
reading 
X 1'8. 
Normal lumi¬ 
nosity curve, 
centre of eye. 
Difference 
of last two 
columns. 
(difference 
x 5T5. 
61 
6839 
2 
36 
4 
•4 
2-57 
59 
6621 
7 
126 
12-5 
-•1 
•51 
57 
6423 
18 
32-4 
33 
+ •6 
3-09 
55 
6242 
36 
64-8 
65 
•2 
1-03 
53 
6074 
49 
88'2 
89-5 
13 
671 
52 
5996 
52 
95-4 
96-5 
IT 
5 - 66 
51 
5919 
54 
97-2 
99-5 
2-3 
11 8 
50 
5850 
54 
97-2 
100 
2-8 
14-4 
49 
5782 
52-5 
94-5 
99'5 
5-0 
25-7 
48 
5720 
50 
90 
97 
7-0 
360 
47 
5658 
46 
82-8 
92-5 
9-7 
49-9 
46 
5596 
41 
73-8 
87 
13-2 
68-0 
44 
5481 
32 
57'6 
75 
17-4 
89 
42 
5373 
23 
43-2 
62-5 
193 
99 
40 
5270 
17 
30-6 
50 
19-4 
100 
38 
5172 
10 
17-5 
35-5 
18 
93 
36 
5085 
4 
7-2 
24 
16-8 
86-5 
34 
5002 
1-0 
1-8 
14-5 
127 
65-5 
31 
4885 
•5 
•7 
6-5 
5-8 
37-7 
28 
4776 
0 
0 
4 
4 
20 6 
A further examination into cases of colour-blindness cannot fail to be interesting, 
and appears to us to throw considerable light on the subject of colour vision. 
Several red and green colour-blind people have been tested in the manner 
described, but the difficulty in many cases of inducing them to note whether the 
observations of extinction were made with the whole eye or the central part only, was 
very great, and there has, therefore, been some uncertainty as to the results. 
We give, however, the results in three cases which may be considered typical, and 
in which the observations appear to have been extremely well made. The first 
(H. R.) is red blind, the second (V. H.) green blind, the third (P.) has monochromatic 
vision. The first two were educated men who understood exactly what they had to 
look for, the last (one of the brothers P. and Q.) was an excellent observer, sharp 
and intelligent, and anxious to help on the experimenter. (See Tables VII., VIII., 
and IX.) 
We are aware that the Yoxjng-Helmholtz theory of vision is open to criticism 
from certain points of view, but we adopt it tentatively as being at least convenient. 
On this theory we should expect, if the monochromatic vision of the third was sup¬ 
posed to consist of the blue (or violet) sensation, that all three of the observers would 
give approximately the same curves in the most refrangible part of the spectrum, since 
in the red blind and green blind this same sensation may be supposed to be existent. 
That the monochromatic sensation is blue, and corresponds to the dominant sensation 
