PEOPESSOE BTJlSrSEN AND DE. H. E. EOSCOE’S PHOTO-CHEMICAL EESEAECHE8. 899 
The several \ allies of I in the 4th column can be expressed, as exactly as the obser- 
vational errors allow, as functions of the zenith-distance (p by help of the simple equation 
I=77-0H-9-275 9 (6.) 
By means of this equation we have calculated and thrown together in the following 
Table, the luminosity of the whole sky for zenith distances varying from 20° to 90°. 
Table VIII. 
0. 
/. 
2- 
- 1 . 
1 2- 
1 . 
1 >- 
1 . 
6. 
1 . 
(j>. 
1 
<p. 
1 . 
2- 
1 . 
0. 
1 
20 
263 
28° 
337 
411 
o 
44 
485 
O 
52 
559 
60° 
634 
o 
68 
708 
76° 
782 
O 
84 
-856 
21 
272 
29 
346 
1 37 
420 
45 
494 
53 
569 
61 
643 
69 
717 
77 
791 
85 
865 
22 
281 
30 
355 
i 38 
430 
46 
504 
54 
578 
62 
652 
70 
726 
78 
801 
86 
875 
23 
290 
31 
365 
39 
439 
47 
513 
55 
587 
63 
661 
71 
736 
79 
810 
87 
884 
24 
300 
32 
374 
40 
448 
48 
522 
56 
597 
64 
671 
72 
745 
80 
819 
88 
893 
25 
309 
33 
383 
41 
457 
49 
532 
57 
606 
65 
680 
73 
754 
81 
828 
89 
902 
26 
318 
34 
392 
42 
467 
50 
541 
58 
615 
66 
689 
74 
764 
82 
838 
90 
912 
27 
327 
35 
402 
43 
476 
51 
550 
59 
624 
67 
699 
75 
773 
83 
847 
The curve (fig. 8, Plate XLIV.) represents the relation between the values of (p as 
absciss® and those of I as ordinates. The points about the curve marked with a cross 
represent the observations made in the morning; those marked by a dot show the 
brightness of the afternoon sky. Prom these it is seen that the illumination efiected 
by the difiuse dayhght for the same zenith-distance of the sun before and after noon 
remains nearlj^ constant, although at these times of day the temperature and humidity 
of the air must differ widely. 
From Table VIII. we can find, for any given zenith distance of the sun, the relation 
between the amount of hght, optically (or visually) measured and taken as unity, which 
falls from a certain spherical surface of sky in the zenith and that (also optically 
measm-ed) falling from the whole sky. The amount of light, chemically measured, 
which falls from this same sui’face of zenith-sky, multiplied by the amount of optically 
measured light which the whole sky gives, must therefore give the chemical action 
which the whole sky would produce on a horizontal unit of surface. According to our 
former defimtion, however, the amount of light falling from a circle of zenith-sky upon 
the unit of area is the chemical brightness of that piece of zenith-sky. We have carried 
out, according to the method already described, a series of determinations of this 
chemical brightness at various times and at different zenith-distances of the sun. Some 
of these observations were made on the 18th of October, 1856, others on July 23rd and 
August 5th, 1858. During all the experiments the sky was perfectly cloudless, and the 
gas had reached the maximum degree of sensibility. The observations are contained 
in the following Table (IX.}. 
