OF THE RETINA TO LIGHT AND COLOUR. 
187 
former reads 30°, no matter what the colour may be ; and that, as the field increases 
about 7|- degrees on the temporal side, the field on the nasal side increases nearly 6° 
—a variation which is in accordance with the table showing the field with variation 
of intensities of the beam (fig. 14). 
17. Luminosities of Colours for Equal Fields. 
From this curve we can calculate, within certain limits, the intensity {i.e., lumi¬ 
nosity) of any colour to give any required extension of field. Suppose, for instance, we 
required to know the luminosity of the whole of the colours of the spectrum at, say, 
30° from the axis on the temporal side, which would give equal fields, we should 
proceed as follows :—Take the height of the ordinate of any colour above (or below) 
the ordinate of 30°, and divide it by 3'75 ; that would give a factor in powers of ^ 
by which the intensity (luminosity) should be diminished, in order to cause the field 
of that particular ray to fall at 30°. Thus, at 48*4 of the spectrum scale, the height of 
the ordinate is 45° (that is, 15° above the ordinate of 30°). Hence, since 15/375 = 4, 
the intensity of the ray would have to be diminished to (^)^ to cause its field on the 
temporal side to fall at 30°. The luminosity of this ray is at the maximum spectrum 
luminosity, or 100, and would thus have to be reduced to 100/16, or 6'25 ; whilst the 
luminosity at scale numbers 38 and 34 would remain the same, viz., 49 and 31 
respectively. On this plan fig. 15 was calculated, which gives the comparative 
luminosities for equal fields, the maximum being made 100. 
Table XIX. 
Scale No. 
Original 
Luminosity. 
Luminosity 
for equal 
fields. 
Scale No. 
Original 
Luminosity. 
Luminosity 
for equal 
fields. 
00 
.3 5 
•58 
36 
40 
77-6 
58 
15 
•56 - 
34 
32 
53-5 
56 
34 
1-GO 
32 
25 
29-0 
54 
55 
1-6 
30 
19 
9-5 
52 
ro 
2-5 
28 
15 
2-5 
50 
94 
8-1 
26 
13 
•7 
48 
100 
16-5 
24 
9-5 
•24 
46 
96 
32-6 
22 
7-5 
•21 
44 
85 
49-4 
20 
6 
•20 
42 
74 
86-2 
18 
5 
•a5 
40 
60 
1000 
16 
4 
•42 
38 
50 
98-0 
This calculation is made on the assumption that the comparative luminosities of 
the colours of the spectrum are the same at 30° on the temporal side as they are 
at 10°. 
2 n 2 
