CHEMICAL INTENSITY OP TOTAL DAYLIGHT IN ECLIPSE OP 1870. 469 
diately afterwards the chemical action of the diffused daylight was determined by the 
other by projecting on the exposed portion of the sensitive paper the shadow of a small 
blackened ball, so placed that its apparent diameter seen from the surface of the paper 
was slightly larger than the sun’s disk. As the instruments were clamped in a constant 
position, it was necessary to vary from time to time the position of the blackened ball 
with reference to the plane of the paper in order to bring its shadow upon the exposed 
portion. We have, however, shown in a former communication that the height of the 
blackened ball from the sensitive paper may safely vary between 140 and 200 millimetres 
without producing any appreciable difference, and in the present experiments its height 
was maintained between these limits. 
The experiments were commenced on Dec. 19th, with the object of determining the 
value of the constant before referred to. As an example of these observations, 65 in 
number, we give the first one of the series : — 
Observed time ll h 15 m a.m. 
Duration of exposure for total daylight 5 s . 
Duration of exposure for diffused daylight 7 s . 
Readings on calibrated strip. Mean. 
I. 100, 97, 100, 97, 104, 96, 97, 96 = 98-3 
II. 93, 93, 97, 94, 95, 92, 93, 94, 95 = 94-0 
The duration of exposures divided into the values in the Table accompanying the 
graduated fixed strip give : — 
Direct Chemical Intensity 0-037 
Diffused Chemical Intensity . . . . 0-101 
Total Chemical Intensity 0-138 
The following Tables contain the results of this preliminary series of observations : — 
Table I. (a). 
December 19tb, 1870. 
Hour. 
Diffused. 
Direct. 
Total. 
Remarks. 
h 
m 
11 
15 
0-101 
0-037 
0-138 
Sky throughout the day per- 
11 
45 
0-112 
0-032 
0-144 
lectly cloudless and of a 
12 
0 
0-113 
0-037 
0.150 
pure blue. Gentle breeze 
12 
15 
0-120 
0-026 
0-146 
from E. Barometer 29*97 
12 
30 
0-108 
0-042 
0-150 
inches. 
12 
47 
0-106 
0-039 
0-145 
1 
5 
0-115 
0-025 
0-140 
1 
15 
0-096 
0-036 
0-132 
1 
57 
0-091 
0-023 
0-114 
2 
6 
0-082 
0-028 
0-110 
2 
22 
0-075 
0-020 
0-095 
2 
36 
0-062 
0-019 
0-081 
2 
57 
0-060 
0-022 
0-082 
3 
6 
0-060 
0-017 
0-077 
3 
21 
0-050 
0-012 
0-062 
3 
36 
0-049 
0-011 
0-060 
3 T 
MDCCCLXXl. 
