176 
CAPTAIN ABNEY ON THE SENSITIVENESS 
employed. In this case the intensity of the light to cause a loss of colour has to be 
increased tenfold, whilst the aperture is diminished to ^ in diameter. In the extinction 
of light the same increase in intensit}^ only requires a diminution to ^ the diameter. 
This seems to show that the stimulus required to produce colour is of a different 
order from that required to produce light. These experiments apply equally to all 
colours, and therefore to the sensations producing them, and if, as in the Heeixg 
theory, there is a black-and-white sensation co-ordinate with the red-green and yellow- 
blue sensations of light, the extinction of the white sensation ought to follow the 
same curve as that of the other sensations. 
The following table gives examples of the extinction of colour. (See fig. 8.) 
Table XT. 
Diameter 
of 
aperture. 
Diameter 
I. 
11 . 
HI. 
Remarks. 
in powers 
of 2. 
Reading. 
Log. 
1 
Reading. 
Log. 
Reading. 
Log. 
0-94 
- -09 
260 
1*76 
350 
0-99 
300 
1-42 
Extinction of -012 
0-724 
- -48 
245 
1-89 
335 
1-12 
280 
1-59 
aperture in I., II., 
0-52.5 
- -93 
220 
211 
310 
1-33 
260 
1-76 
III. were in Logs. 
0-35 
-1-52 
210 
2-19 
295 
1-46 
235 
1-98 
2-62,1-85, and 1-25 
0-17 
-2-56 
170 
2-54 
255 
1-80 
205 
2-235 
respectively; this 
O-OSG 
-3-.56 
125 
2-925 
210 
2-19 
170 
2‘54 
would make the 
0-036 
-4-81 
75 
3355 
1.55 
2-66 
120 
2-97 
extinction of -94 
0-012 
' 
-6-4 
10 
3-91 
100 
3-14 
60 
3-48 
aperture that 
of the colour ex¬ 
tinction -2 for I., ' 
11., and for III., 
In Colour 
Photometry, Part ; 
111., they were ; 
and ~ respec- ‘ 
tively. 
Nos. I. and II. are the same ray (44 on the scale), but with different intensities. 
No. I. was measured by myself, and No. II. by Corporal Attewell. No. III. was 
read by myself, and was D in the spectrum or scale No. 50'6. It may be remarked 
that with the small apertures the extinction of colour in the red was impracticable, 
as the extinction of lisfht and colour took place together, as it should do according to 
other experiments. 
12. Colour Fields and Perimeters. 
An enquiry was next undertaken as to the variation in the extent of colour fields 
under different conditions of light. The question of colour fields is one regarding 
which much has been written, and experiments on the subject have been numerous, 
but with one or two exceptions it is believed that these latter have been carried out 
