S94 PEOFESSOE BUNSEN AND DE. H. E. EOSCOE’S PHQTO-CHIAnCAi EESEAECHES. 
saturated with acid remains, the breadth of which should be about 1 to 2 mi l l i m etres, 
and the diameter from 5 to 6 millims. 
'Wg have already mentioned that we were only able to measure the chemical action 
produced by a small circle of sky at the zenith. In order to be able to determine the 
action effected by the light from the whole visible heavens, it was in the fii'st place 
necessary to compare, by means of the photometer just described, the amount of light 
which a point on the earth’s surface receives from the whole sky, with that which the 
same point receives from a measured circular portion of sky at the zenith. the 
quantity of light which emanates from the measm’ed portion of sky in the zenith is 
from 300 to 800 times less than that reflected from the whole sky, and as nith this 
photometer we are unable to measure such large differences in the amount of light, it 
was necessary to employ some known fraction of the total diffuse light of day. ffor this 
purpose a hollow hemispherical metallic cap (flg. 7, Plate XLIII.) was made use of. This 
cap, which is painted black, can be placed concentric with the diaphragm («, fig. 4), on 
the plate cc, and is perforated at regular distances with 184 equal-sized holes, the 
diameter of which was accurately determined. In order to measure the light from a 
circle of sky at the zenith of known magnitude, the blackened tube (fig. 6, Plate XLIII.) 
was employed ; this tube is furnished at its upper end with a semicircular moveable 
disk, similar to the one already described as placed at the lower end of the photometer- 
tube (b, fig. 6), and can be fixed on the plate cc, so that the ring of the paper dia- 
phragm (a) falls exactly below the centre of the divided semicircle. 
The observations are commenced by placing the blackened cap on the plate over the 
diaphragm ; the diffuse light falls through the holes upon the diaphragm, and the 
divided disk in the tube (b) is moved until the ring on the paper, seen through the eye- 
piece b, disappears. The amount of light which under these circumstances is reflected 
from the sky upon the diaphragm we will call 1. Let the total surface of the hemi- 
spherical cap be Qi, and that portion which has been removed by the holes be Q; then 
the amount of light which would have fallen on the cliaphragm from the whole sky if 
the cap had not been present is equal to If the cap be remo^^ed, and in its stead 
the tube (fig. 6) placed on the plate, without otherwise altermg the aiTangement, and 
then the divided circle on the ujtpcT end of the tube moved until the ring disappears, the 
diaphragm (ci) is again illuminated "with the intensity 1. Let ^ represent the fraction 
of the total hemisphere from which the light falls through the open sector of the tube 
upon the diaphragm, and let g'l represent the area of a given circle of sky at the zenith 
expressed in the same units as g ; the amount of light which would faU upon the dia- 
phragm from the whole circle of sky §■! is ; and hence the relation between the light 
^ . Q o 
from the whole heavens and that from the circle of sky therefore, we 
know the amount of chemical action (w) which the light reflected from a cu’cle of sky 
at the zenith effects on a given point, the action (W) which the light from the whole 
