ON COLOUR PHOTOMETRY. 
559 
re-combined white patch was thus determined. As the value of the white light 
reflected from M to the end of the box was known from the first observation, the 
luminosity of the D light so reflected was calculated. The luminosity of the D light 
having been found, that of all the other rays was calculated from the luminosity curve 
derived from observations made with the central portion of the retina (see fig. 33, A), 
as it was with this part that the observations now being described were made. 
The actual value of each ray when the colour disappeared was calculated from the 
aperture of the sectors. 
Table II.—Limit of Colour Vision. 
Scale number. 
Wave-length. 
Mean reading of 
the colour limit 
of the spectrum 
D, being 1 amyl 
lamp m 10 -J 00 ths. 
Luminosity of 
the ordinary 
spectrum. 
Luminosity of the 
rays when each 
colour disappears, 
each ray having the 
original luminosity 
of 1 amyl lamp in 
1 f| 1C! 
lOOOOO 
61 
6839 
120 
4 
48-0 
60 
6728 
67 
7 
46-9 
58 
6520 
26 
21 
54-6 
56 
6330 
13 
50 
65-0 
54 
6152 
9-5 
80 
76-0 
52 
5996 
9-0 
96 
86-4 
50 
5850 
9-0 
100 
90-0 
48 
5720 
9-0 
97 
87-3 
44 
5481 
9-5 
75 
71-3 
40 
5270 
10-5 
50 
52-5 
36 
5085 
12-5 
24 
30-0 
32 
4924 
18 
8-5 
15-3 
28 
4776 
32 
40 
12-8 
24 
4639 
55 
1-8 
120 
20 
4517 
90 
1-08 
97 
16 
4404 
160 
•70 
11-2 
12 
4296 
250 
•45 
110 
8 
4197 
400 
•26 
10-4 
4 
4106 
700 
T4 
9-8 
In fig. 37 the continuous curve is constructed from these observations,* and the 
dotted curve B is that derived from curve A, supposing that each ray had an original 
luminosity of one amyl light at the distance of 1 foot. 
It will be seen that the colour of the central portion of the spectrum is discernible 
with much greater reduction of light than is that of the extremities. This accounts 
for the fact that objects illuminated by moonlight appear of a greenish hue. The 
light from the full moon, as is well known, is somewhere about half a million times 
less bright than that of the sun, or about yuo of an amyl lamp at 1 foot. The figure 
* The extreme left end being plotted to a different scale so as to bring it within the paper. 
3 z 2 
