314 
MESSES. EOSCOE AND THOEPE ON THE EELATION 
The results are more clearly seen in the following Table. 
Table II. 
Time. 
Mean 
Mean chemical intensity. 
Number of 
Diff. 
altitude. 
Sun. 
Diffused. 
Total. 
observations. 
h m 
6 0 A.M. 
10° 23 
o-ooo 
0 041 
0-041 
3 
+ -003 
6 0 p.m. 
9 28 
o-ooo 
0-035 
0-035 
12 
7 0 A.M. 
19 14 
0-020 
0-061 
0-082 
8 
-0-025 
5 0 p.m. 
20 09 
0-025 
0-062 
0-087 
10 
8 0 A.M. 
30 33 
0-052 
0-097 
0-146 
11 
-0-005 
4 0 p.m. 
31 54 
0-053 
0-104 
0-157 
11 
9 0 A.M. 
41 43 
0-102 
0-114 
0-216 
11 
+ 0-001 
3 0 p.m. 
42 43 
0-099 
0-115 
0-214 
11 
10 0 A.M. 
53 01 
0-149 
0-130 
0-279 
13 
+ 0-025 
2 0 p.m. 
53 18 
0-108 
0-120 
0-228 
6 
110 A.M. 
61 09 
0-204 
0-131 
0-335 
13 
+ 0-009 
1 0 P.M. 
61 07 
0-184 
0-133 
0-317 
11 
12 0 Noon. 
64 14 
0-221 
0-138 
0-359 
11 
The numbers contained in columns 3 and 4 of Table II. give the relation between 
direct sunlight and diffuse daylight. These sets of numbers, expressed graphically in terms 
of the altitude, are seen on Plate XXX. fig. 3 ; the dotted curve indicating the intensity 
of the diffused light, the black curve that of the direct sunlight. These two curves inter- 
sect at an altitude of 50°, at which elevation, therefore, the place of equal chemical illu- 
mination occurs for a surface placed in the plane of the horizon. 
The fact that the curve of direct sunlight cuts the base line at 10° bears out the con- 
clusion which one of us has already announced, namely, that at altitudes below 10° 
direct sunlight is robbed of almost all its chemically active rays. 
The curves (fig. 1) show the daily march of chemical intensity at Lisbon, as a mean 
of all the observations, compared with that at Kew for the preceding August, and at 
Para for the preceding April. The number representing the mean chemical intensity at 
Kew is 94-5, at Lisbon 110, and at Para 31 3' 3 ; light of the intensity 1*0 acting for 
twenty-four hours being taken as 1000. 
If we now arrange the observations according to the sun’s altitude, we have : — 
Table III. 
Number of 
observations. 
Mean 
altitude. 
Chemical intensity. 
Sun. 
Sky. 
Total. 
15. 
o / 
9 51 
o-ooo 
0-038 
0-038 
18. 
19 41 
0-023 
0-062 
0-085 
22. 
31 14 
0-052 
0-100 
0-152 
22. 
42 13 
0-100 
0-115 
0-215 
19. 
53 09 
0-136 
0-126 
0-262 
24. 
61 08 
0-195 
0-132 
0-327 
ii. 
64 14 
0-221 
0-138 
0-359 
