888 PEOFESSOE BUNSEN AND DE. H. E. EOSCOE’S PHOTO-CHEmCAL EESEAECHES. 
K.*=l-777 for the loss of light by the reflexion from mirror, 
li =1*450 for the loss of light from the mica-plates. 
N =1*031 for the loss of light from the glass sides of insolation-Tessel. 
n =2*249 for the reduction of the observed divisions on scale No. 2 to degrees of light. 
Column V. gives the required mean brightness calculated from formula (3.) ; column YI. 
contains the magnitude of the observed surface compared with that of the hemisphere 
taken as unity. 
Table III. 
No. of 
experiment. 
I. 
d . 
IL 
r . 
III. 
w . 
IV. 
1. 
Y. 
H. 
VI. 
1 . 
m. 
0*0590 
m. 
2*2^5 
10*89 
65*1 
0*741 
0*0000878 ! 
2. 
0-0359 
2*225 
4*04 
24*1 
0*744 
0*0000325 1 
3. 
0*0590 
2*225 
10*59 
63*3 
0*720 
0*0000878 
4. 
0*0530 
2*225 
8*71 
52*0 
0*733 
0*0000708 
5. 
0*0590 
2*225 
10*60 
63*3 
0*721 
0-0000878 
6 . 
0*0359 
2*225 
3*89 
23*2 
0*715 
0*0000325 1 
From these numbers it is seen that, within the limits of experimental error, the por- 
tions of sky near the zenith are equally bright ; hence that the chemical action efiected 
by portions of zenith-sky not exceeding 0*00009 of the total heavens, may be taken to 
be directly proportional to their apparent magnitudes, when the sun does not materially 
alter its position. 
The determinations hitherto considered all depend upon the comparison of the light 
which has to be measured with that of a constant luminous source. In many cases, 
however, it is of importance to express the chemical action of light, not only compara- 
tively in units of light or in degrees of light, but also absolutely, that is, in units of 
time and sggace. Such an absolute measure can be obtained from the observations of 
our instrument when the following data are given 
D=the volume of hydrochloric acid, measured at 0° C. and 0™*76, formed by the imit 
of light ; 
^=the thickness of sensitive gas (reduced to 0° and 0*76) through which the light 
passed ; 
2 '= the surface-area of the insolated gas; 
C!:=the coefficient of extinction of the chlorine and hydrogen for the light employed; 
I =the number of observed units of light in the time t. 
When the values of these quantities are known, the volume of hydrochloric acid which 
would be formed in the time t, by the rays falling perpendicularly on the unit of sui*face, 
\ih~^ , or if the light had been completely extinguished in passing through an inde- 
finitely extended atmosphere of dry chlorine and hydrogen gas, is found from the 
equation 
\ — 
q 1 — 10-“* 
* The value of E was deterinmed by a special experiment with candle-light. 
( 4 .) 
