INTENSITY OF THE CHEMICAL ACTION OF TOTAL DAYLIGHT. 
663 
one fifth of a second. A large number of observations were made with this instrument, 
and the mean reading of the interval taken as the duration of exposure for each one 
(see Table infra). In order to check these times, the duration of the longer intervals 
was also ascertained by counting with a watch whose seconds’ hand indicated quarter 
seconds ; and these coincided with the times observed with the chronograph. 
Interval. . 
No. 1. 
No. 2. 
No. 3. 
No. 4. 
No. 5. 
No. 6. 
No. 7. 
No. 8. 
No. 9. 
No. 10. 
2-2 
2-8 
3-8 
5-2 
6-7 
9-5 
10-8 
17-3 
20-2 
31-4 
2-0 
3-0 
2-8 
5-3 
7-0 
8-7 
10-3 
16-2 
20-6 
32-0 
1-7 
3-8 
3-0 
6-0 
7-0 
8:3 
11-0 
17-8 
19-7 
32-4 
2-9 
3-1 
4-3 
5-3 
6-8 
9-9 
10-0 
16-9 
19-4 
31-1 
1-7 
2-0 
3-1 
4-8 
6-7 
9-7 
9-7 
17-2 
20-4 
30-7 
2-7 
2-fi 
3-9 
5-2 
6-7 
9-0 
10-2 
18-1 
20-5 
31-7 
2-9 
1-9 
3-6 
6-1 
6-8 
9-5 
10-0 
16-6 
20-2 
31-4 
3-0 
4-0 
5-2 
7-0 
9-2 
10-3 
17-0 
20-5 
31-1 
4-1 
5-2 
6-3 
10-0 
9-9 
16-7 
20-2 
31-6 
4-5 
6-9 
9-2 
10-1 
17-9 
20-5 
32-0 
5-3 
7-1 
9-7 
10-6 
16-6 
19-7 
32-6 
5-3 
7-0 
9-4 
10-1 
17-0 
20-1 
32-4 
6-5 
9-4 
10-3 
16-9 
6-9 
10-1 
10-0 
7-0 
10-4 
10-1 
7-0 
10-1 
10-1 
6-9 
9-7 
10-3 
10-2 
Mean .... 
2-3 
2-8 
3-6 
5-3 
6-9 
9-6 
10-2 
17-1 
20-2 
31-7 
V . Determination of the Coefficient for the Deflection and Absorption due to glass 
cover. 
In order to keep the disk of sensitive paper dry and to preserve the insolation-appa- 
ratus from damage by rain, it is covered during wet weather with a glass shade, chosen 
of such dimensions that the exposed disk of paper lies as nearly as possible in the 
centre of the hemisperical top of the shade. For the purpose of determining the value 
of the coefficient representing the loss of light caused by this glass covering, simulta- 
neous observations of the intensity of total daylight were made with the hand-insolator 
and with the self-registering instrument covered by the glass shade. From the equa- 
tion I A =I e x const (where \ h signifies the intensity obtained by the hand-instrument, 
and I e that obtained by the self-recording instrument covered with the shade) the value 
of the coefficient can be ascertained. The following experiments give the value of the 
coefficient for the glass shade used in the determinations which follow. A similar 
series of observations must be made for every shade employed. 
4 u 
MDCCCLXXIY. 
