926 PEOFESSOE BIJlSrSEN AXD DE. H. E. EOSCOE’B PHOTO-CBEMICAL JRESEAECHES. 
be different. The order and degree in which the chemical rays are absorbed, may be 
obtained by repeating the observations according to the above method from honr to 
hour during a whole day. Such a series of experiments we have unfortunately as yet 
been unable to execute, owing to the variability of the weather in our latitudes. One 
Tery imperfect series of observations we can, however, quote, and they sufB.ce to show 
that the relation between the chemical action of the spectral coloirrs is perceptibly 
altered when the thickness of air through which the rays pass changes jhom 9647 to 
10,735 metres. 
These experiments were likewise made on August the 14th, 1857, in the short space 
of time from 9^ 44“ to lO’^ 19“ a.m., and gave the follovraig numbers reduced to the 
zenith-distance (42° 46'), corresponding to 10^ 0“ a.m. They were, however, made with 
a bundle of rays of a different thickness from the former experiments, and therefore 
cannot be compared with those. 
Xd. 
Time. 
IPortioii of.£psctrum. 
nektive 
chemical 
action. 
1 
h m 
9 44 A.M. 
From ^ GH to I ............ 
14-5 
.2 
9 48 A.M. 
From Ng to E^ 
10-1. 
3' 
9 .54 A.M. 
From A E2S to i ST 
2-4 
4 
9 59 A.M. 
From ^ ST to U 
0-0 
5 
11) 4 A.M. 
From G to 4 GH ..... 
13-0 
6 
10 8 A.M. 
From F to i FG 
7-1 
7 
10 11 A.M. 
From b to 4 FG 
3‘2 
8 
10 15 A.M. 
From 4 DE to f EF 
0-4 
These observations are represented by the lines 5 5 5 (fig. 19, Plate XLYII.) in wlrich, 
it must be remembered, a unit different fi'om that of the other experiments is employed. 
From this it is seen that the relation of the chemical action of the spectrum from the 
line E to the line H. undergoes a considerable alteration when the rays have to pass 
through a column of air 10,735 metres in height instead of 9647 metres. 
An extended series of measurements of the chemical action of the several portions of 
the solar spectrum under various conditions of atmospheric extinction, may prove of 
great interest, if, as we can irow scarcely doubt, the solar spots appear at regular inter- 
vals, and our sun belongs to the class of fixed stars of variable illuminating power. It is 
possible that such observations, made during the presence and dming the absence of 
the solar spots, may give rise to some unlooked-for relations concerning the singular 
phenomena occurring on the sun’s surface. Whether, however, the atmospheric 
extinction can ever be determined with sufficient accuracy to render risible the altera- 
tion in the light which probably occurs with the spots, is a question which can only be 
decided by a series of experimental investigations which must extend far beyond the 
scope of any single observer. 
