PEOFESSOE BUNSEN AND DE. H. E. EOSCOE’S PHOTO-CHEMICAL EESEAECHES. 
897 
Table VI. 
I. 
II. 
III. 
4 '- 
IV. 
I. 
1 
h m 
5 38 A.M. 
88-8 
762 
2 
6 8 A.M. 
86-0 
738 
3 
7 25 A.M. 
74-2 
637 
4 
8 52 A.M. 
56-0 
481 
5 
9 32 A.M. 
50-4 
432 
6 
10 9 a.m. 
47-2 
405 
7 
10 43 A.M. 
42-0 
360 
8 
11 26 A.M. 
37-8 
324 
9 
0 1 P.M. 
35-8 
307 
10 
0 32 p M. 
38-4 
330 
11 
0 57 P.M. 
42-0 
360 
12 
1 24 P.M. 
47-4 
407 
13 
2 4 P.M. 
50-0 
429 
14 
2 38 P.M. 
53-0 
455 
15 
3 21 P.M. 
60*8 
522 
16 
3 57 P.M. 
68-8 
590 
17 
4 42 P.M. 
76-0 
652 
18 
5 20 P.M. 
79*6 
683 
19 
6 2 P.M. 
88-0 
755 
The amounts of light which fall upon the diaphragm-ring and are arranged in column 
IV. of the foregoing Table, in no way represent the mean brightness of the whole sky 
as compared with the respective brightnesses of the zenith, for it is only the zenith rays 
which faU perpendicularly upon the unit of surface ; the rays from other points of the 
sky with a greater zenith-distance fall upon the diaphragm with a larger angle of 
incidence. This relation between the luminosity of the zenith and the mean luminosity 
of the whole sky, could be easily found if, instead of the perforated cap used, another 
were employed in which the number or area of the holes was in the inverse proportion 
of the cosines of their distance from the vertical. 
The amount of chemical illumination which a horizontal unit on the earth’s surface 
receives from the whole heavens, depends upon the height of the sun above the horizon? 
and upon the constitution of the atmosphere. If, when the sky is cloudless, the atmo- 
sphere undergoes considerable change in its transparency, a long series of determinations 
made at different times of the year would be necessary before we could hope to arrive 
at any general expression for the atmospheric extinction and dispersion of the chemical 
rays. Fortunately, however, we learn from Professor Seidel’s classical research on the 
Brightness of the Fixed Stars *, that the atmospheric extinction varies so slightly when 
the sky is cloudless that the differences may be altogether neglected. In a lecture 
given at Munich ‘f*, Seidel says, “It has fortunately been proved, although one could 
scarcely have expected it, that the variations in the amount of transparency of the air on 
* Abhandl. der Eon. Bayer. Acad. d. Wiss. 2te Classe, Bd. vi. Abth. 3. 
t Wissenschaftlicbe Vortrage, gebalten zu Munchen im Winter, 1858, p. 301. Braunscbweig bei Vieweg 
und Sohn. 
