114 
SIR W. DE W. ABNEY AND PROF. W. WATSON ON 
values varying inversely as the square of the diameter. The results are therefore in 
marked contradiction to those obtained by Loeser and Piper. 
3. Luminosity at the Threshold. 
In the preceding section we have considered the threshold values measured in 
terms of the objective energy of the radiation required to produce the sensation of 
light. We have now to consider what are the threshold values measured in terms of 
subjective light. In other words, in place of using the energy curve to give the 
distribution of the radiation in the spectrum, we have to use the luminosity curve, that 
is the curve which gives the relative brightness of the different parts of the spectrum 
considered as light. Now the shape of the normal luminosity curve varies with the 
intensity of the spectrum ; the curve obtained with a bright spectrum when the cones 
are principally, if not exclusively, functionary, differs markedly from that obtained 
with a very feeble spectrum when the rods are alone operative. In the case of 
threshold vision we have to do with rod vision except in the red and for the fovea.1 
values obtained by observers of class I., and hence the luminosity curve to be employed 
to give the relative brightness of the different parts of the spectrum is that obtained 
at extremely low intensities or at any rate where the rods are alone operative. Such 
a curve can be obtained by a person having normal colour vision by the use of a very 
feeble spectrum but the observations are very difficult. Hence it is better to make 
use of the fact that the luminosity curve of a person who has monochromatic colour 
vision corresponds to rod vision only. One of us has determined the luminosity curves 
of three monochromats and we have taken the mean of the three as giving the rod 
luminosity curve. The observations were taken with the arc as a source of light but 
by means of the energy curves given in fig. 3 we have calculated the corresponding 
curve for the Nernst light. The values both for the arc and for the Nernst as light 
sources are given in Table IV. 
Table IV.—Luminosity Curves for Monochromat (Rod Vision). 
SSN. 
Luminosity. 
SSN. 
Luminosity. 
Arc. 
Nernst. 
Arc. 
Nernst. 
52 
8-8 
14-2 
30 
42 • 2 
37-5 
50 
19-2 
28-1 
28 
32-1 
27'5 
48 
35-5 
47-9 
26 
24-4 
20-4 
46 
56-4 
71-4 
24 
18-6 
15-1 
44 
74-5 
88-0 
22 
13-7 
11-1 
42 
86'1 
95-3 
20 
10-1 
8-1 
40 
87-8 
93-8 
18 
8-1 
6-4 
38 
84-8 
86-0 
16 
5'9 
4-6 
36 
78-8 
78-0 
14 
4-7 
3-6 
34 
69-3 
66-2 
12 
3-2 
1-6 
32 
56-5 
51 • 5 
10 
2‘3 
