178 
CAPTAIN ABNEY ON THE SENSITIVENESS 
The hemisphere was furnished with a chin and cheek rest, which would move round 
a vertical axis. It was divided internally into degrees. The eye was directed to any 
part of the surface by means of a small phosphorescent bead at the end of a stick; 
and a small electric lamp, which could be switched on by a simple movement of the 
liand, gave light sufficient to read the position occupied by the bead at any desired 
instant. The intensity of the light illuminating the ground glass was altered by 
any of the four methods mentioned above. The annulus was usually employed to 
effect the alteration, and it could be rotated at the will of the obseiwer by a long 
handle attached to the rack and pinion motion of the rotating gear. 
13. Similarity of Fields for Different Colours. 
The order in which the experiments were made will not be followed, for, as a 
matter of fact, amongst those to be first described some were among the latest, and 
others among the earliest made. It was essential to know whether the fields for each 
colour were of the same form when the illumination was so adjusted that one point 
in a field of one colour coincided with one point in the field of a different colour. 
The following two sets of observations made by myself, and the succeeding ones 
made by one of my assistants (W. B.), will give the answer to the inquiry. 
An aperture of '525 inch subtending an angle of 2° 30' was inserted behind the 
ground glass, and the light falling on the eye when D was the ray selected, was 4'5 
amyl-acetate lamps at I foot. (In future tliis light will be designated as AL, and 
this particular illumination would be 4’5 AL.) 
The following rays were used to illuminate the aperture: red lithium (XG/05) 
D (X 5892), a ray having the standard scale number ‘36 (X 5085), and the blue lithium 
ray (X4603). These had respectively the luminosities of '3, 4’5, 2’1, and '4 AL. 
The measures were made with the right eye (see fig. 9). 
Table XII. 
Angle of field 
in degrees. 
Extent of fields in degrees. ' 
Red Li. 
D. 
SN 36. 
Bine Li. 
0 
35 
36 
24 
40 
.301 
37 
40 
27 
47 i 
60 
47 
50 
33 
57 
90 >T. 
55 
57 
38 
65 
120 
51 
53 
36 
60 i 
150 J 
41 
43 
29 
50 1 
180 
34, 
36 
25 
40 
1500 
35 
36 
26 
40 
1 120 
37 
38 
27 
45 
1 90 
U- 
1 40 
42 
28 
49 
60 
38 
40 
27 
45 
30, 
' 34 
1 
;l() 
25 
42 
