924 PEOPESSOE BUNSEN AND DE. H. E. EOSCOE’S PHOTO-CIEEAnCAL EESEAECHES. 
disposal. The figure (fig. 19, Plate XLVII.) contains a copy of Mr. Stokes’s map, with 
the distances measured by him, and letters given according to his notation. We have 
divided the space between the letter A in the red to the last ray Stokes observed, M in 
the lavender rays, into 160 equal parts, and we represent the position and breadth of 
the bundle of rays which effected a given action upon the insolation-vessel as follows : — 
If a bundle of rays lying between the abscissae 20’5 and 34 in fig. 15 had to be represented, 
we should call the edge of the bundle towards A, ^ DE, and that towards M EG, 
whilst the middle of the bundle, or the portion of the spectrum which produces the 
action, we call “ DE to ^ EG.” The breadth, of this bundle of rays, in which the inso- 
lation-vessel was completely bathed, was of the total length of the spectrum. 
The following Table gives the direct results of a series of observations made by per- 
fectly cloudless sky at Heidelberg, on the 14th of August, 1857, under a barometric 
pressure of 0“’7494. The first column gives the numbers of the observations in the 
order in which they were made ; column II. the times of observation in true solar time ; 
column III. the portion of spectrum under examination ; and column IV . the action 
corresponding to this portion. 
I. 
II. 
III. 
IV. 
1 
h 
10 
m 
54 A.M. 
From f GH to I 
48-80 
2 
10 
58 A.M. 
From i DE to E ... 
1-27 
3 
11 
4 A.M. 
From C to 4 DE 
0-47 
4 
11 
8 A.M. 
From Nj to QR..... 
18-28 
5 
11 
13 A.M. 
From 4 RS to ST 
2-03 
6 
11 
41 A.M. 
From f ST to I UV 
1-27 
7 
11 
47 A.M. 
From N^Q to -I- RS ... 
From -f ST to UV.. 
11-73 
8 
11 
50 A.M. 
1-02 
9 
11 
54 A.M. 
From -I IM*! to N 4 ...... 
37-87 
10 
11 
57 A.M. 
From Hi to f IMi 
57-42 
11 
0 
1 P.M. 
From III to 4 IMi •••••• 
52-30 
12 
0 
4 P.M. 
From 4 GH to H 
61-38 
13 
0 
7 P.M. 
From 4 FG to G 
27-64 
14 
0 
16 P.M. 
From 4 FG to G 
28-74 
15 
0 
20 p.M. 
From 4 DE to F 
1-39 
16 
0 
25 p.M. 
From 4 N 4 Q to 4 RS ... 
13-19 
17 
0 
32 p.M. 
From 4 N 4 Q to 4 RS ... 
12-41 
18 
0 
40 p.M. 
From G to 4 GH 
53-78 
19 
0 
42 p M. 
FromlGH to H 
68-74 
20 
0 
45 p.M. 
From 4 GH to I 
53-9 
If the fraction of the unit amount of incident light which is reflected from the mirror 
of the heliostat at the commencement and at the end of the series of experiments be 
calculated, we get the numbers 0‘644 and 0'642, which differ so slightly, that the 
variations brought about by the reflexion may be neglected without overstepping the 
observational errors. At the times of observation on the 14th of August, 1857, the 
sun’s zenith-distance was as follows : — - 
li in o 1 
At 10 54 A.M. 37 35 
At 0 0 A.M. 35 13 
At 0 45 p.M. 36 16 
