OF SUNLIGHT THROUGH THE EARTH’S ATMOSPHERE. 
7 
at a little distance to one side of the source of light and the reflected beam was 
made to illuminate the shadow falling on the white square, and itself to cast a shadow 
on the dark square. Thus each square was illuminated by the same light, but 
coming from different points, and the two would be caused to be of equal darkness 
by placing rotating sectors opening and closing at pleasure in front of one or other 
of the beams. The amount of light cut off gave a measure of the darkness. Thus, 
in one case, when both squares were white the sectors had a total aperture of 80° to 
make them equally luminous ; when a grey platinum square was substituted for one of 
them, the aperture of the sectors was 45°. The grey surface, therefore, only reflected 
of the light that the white surface reflected. The light to these squares was admitted 
through an aperture in the front of a closed box, the illumination being judged through 
an aperture fixed at one corner. The whole apparatus was placed in a darkened and 
blackened room in which the sole light was that used for the measures. 
Fig. 2. 
The two following scales of blackness, or, perhaps more accurately, of whiteness, 
were obtained by this plan, one or other of which will apply to all the measurements 
given in the paper :— 
Comparative 
intensities of 
light. 
Curve I. 
Curve II. 
Amount of white 
reflected. 
Amount of white 
reflected. 
1 
100 
100 
2' 
99-3 
99 
2^ 
97-2 
96 1 
2 
93-9 
91 i 
2*- 
89-5 
84-5 
2^ 
84-6 
77 
4 
78 
69 
2^ 
71 
60-5 ! 
2"- 
64 
51-7 1 
8 
57 
43-2 
2’ 5 - 
50 
35-5 I 
2” 
43 
28-5 
16 
37 
22-7 
2” 
31 
17-5 
2’^‘- 
.25-5 
13-3 
.82 
21 
9-8 
2- 
17 
2- 
13-5 
64 
10-5 
1 
