OF THE RETINA TO LIGHT AND COLOUR. 
191 
Table XXII. 
i Diameter 
of 
Angle 
Diameter 
of 
aperture 
in powers 
of 2. 
Red lithium. 
D. 
41-7. 
Bine lithium. 
; nperture 
in 
inches. 
sub¬ 
tended. 
Tem¬ 
poral. 
Nasal. 
Tem¬ 
poral. 
Nasal. 
Tem¬ 
poral. 
Nasal. 
Tem- 
2 :)oral. 
Nasal. 
0-94 
O < 
4 18 0 
-0-09 
42 
32 
48 
35 
38 
28 
50 
37 
0-525 
2 30 0 
-0-93 
37 
28 
43 
32 
33 
25 
47 
34 
0-35 
I 34 0 
-1-52 
35 
26 
39 
29 
31 
23 
42 
31 
0-17 
0 49 0 
-2-56 
29 
22 
34 
25 
25 
18-5 
37 
27 
0-086 
0 25 0 
-3-56 
25 
17-5 
29 
21 
20 
15 
32 
23 
0-036 
0 10 0 
-4-8 
19 
14 
25 
18 
b.s. 
12 
27 
21 
0-012 
0 3 30 
-6-4 
b.s. 
12-5 
20 
15 
b.s. 
9 
20 
15 
b.s. is blind spot where measures are impracticable. 
This table when plotted gives a diagram, fig. 17, which shows that between 
apertures subtending 4° 28' and 10' (the power of ^ being taken for the scale of 
abscissa), the fields decrease in extent and are practically straight lines. On the 
temporal side for each diminution in aperture to ^ diameter the diminution in field is 
5° and on the nasal side 4°. The diminution in field for a diminution of ^ the intensity 
oflight.^ it will be remembered, is 7'5" on the temporal side and 6° on the nasal side. 
Fig. 17. 
The diminutions in field thus bear the same ratio to one another, viz., 5 : 4. This 
might be expected, but the author was by no means prepared to find that it could 
be measured so closely as it has been. We thus arrive at the result that diminution 
in aperture is equivalent to diminution in intensity of light. When the apertures 
used were greater than the largest given in the table scarcely any alteration of the 
field was obtained. We may take it that any aperture subtending more than 5° 
will give the same field. With apertures between 5° and 3° the field will only 
slightly diminish. Referring to the table of extent of field for the whole spectrum, 
we shall find that the measured field for an aperture of '086 inch agrees with the 
above determination very closely, taking into account the illumination. 
