172 
CAPTAIN ABNEY ON THE SENSITIVENESS 
The method adopted was to place pins with heads painted with Balmain paint at 
every 5° from the central line joining the illuminated aperture and the position 
occupied by the eye. The paint was very feebly phosphorescent, and only just suffi¬ 
cient to fix the centre of the eye at the required angle from the object. The results 
of two experiments, red and white light (paraffin), at 10° are given. It appears from 
these that at this angular distance the extinction of all light from the red takes 
l^lace when the light is about one-third brighter than is required for the centre of 
the eye. With the paraffin light it is somewhat less. With green light about E, 
and ^vith blue at the lithium line, the necessary reduction of the light is greater than 
for the centre of the eye, a result already shown in “ Colour Photometry,” Part III. 
Table IX. 
Aperture. 
. 
Angle. 
2-e 
Red light. 
White light. 
Direct. 
10 ° from axis. 
Direct. 
10 ° from axis. 
0-940 
0 
1 
57 
H 
0 
0-09 
275 
255 
305 
290 
0-724 
I 
30 
0 
0-48 
252 
230 
270 
265 
0-525 
1 
.5 
0 
0-93 
225 
204 
265 
240 
0-420 
0 
52 
35 
1-25 
217 
195 
252 
230 
0-350 
0 
43 
43 
1-52 
195 
174 
235 
220 
0-300 
0 
37 
33 
1-74 
185 
162 
215 
200 
0-170 
0 
21 
17 
2-56 
152 
125 
174 
157 
0-086 
0 
10 
46 
.3-56 
93 
75 
118 
105 
There is a further falling-off of sensitiveness at greater angles than those shown in 
the tables, but the extinction is very difficult to make with certainty. 
10. Lv.muiosity of the Light coming through cUfevent Apertures. 
The next point investigated, but Without any great degree of detail, was themom- 
pamtive luminosities of the same light coming through two apertures of different 
diameters. The method adopted was as follows :—^The ground glass was illuminated 
uniformly with the light to be tested, and two apertures 
cut in a black mask were placed in contact with it, as 
shown. Sectors were placed close behind the larger aper¬ 
ture, and rotated with angular apertures of any desired 
amount. In front of the slit of the spectrum the annulus 
was placed so that a regular diminution of the light could be effected. The sectors 
having been set at 90°, the light coming through the bigger aperture was diminished 
to half. As the light coming through the" small'aperture is'extinguished Ibng'before 
that coming through the larger one, there must be sOme intensity of light when'the 
two apertures will appear equally bright to the eye in the extinction box. ‘ ’Thedight 
