PEOFESSOE BUNSEN AND DE. H. E. EOSCOE’S PHOTO-CHEMICAL EESEAECHES. 891 
II. CHEMICAL ACTION OE DIFFUSE DAYLIGHT. 
It would be extremely difficult to measure directly, by means of our chemical photo- 
meter, the amount of hght which a point of the earth’s surface receives fro m the whole 
atmosphere by dispersion. Experiments of this kind can of course only be made in the 
open air, either at an elevated situation Avith an extended horizon, or in the middle of a 
large plain removed from all objects which could obstruct or reflect lig ht- When, in 
addition to this, we remember that the instrument with which the observations are 
made is so sensitive that a violent explosion occurs when a feeble ray of sunlight falls 
upon the apparatus, it will not appear surprising that we endeavoured to obtain the 
wished-for result in another manner. For this purpose we determined chemically in 
absolute measure, the action of the rays falling from a measured portion of cloudless, 
sky situated near the zenith, and then compared the visual luminosity of this same por- 
tion of zenith-sky with that of the total heavens. 
In order to be able to make this comparison, we had to carry out a somewhat lengthy 
series of experiments, which, as they form the basis of our present measurements, we 
must consider in detail. 
As the light ft-om certain zones of sky varying with the sun’s altitude is polarized, 
and may therefore disappear on reflexion, it is impossible, in these determinations of 
the visual luminosity of the atmosphere, to employ any arrangement of mirrors. It 
appeared at first sight, that instead of reflecting the light it would be most convenient 
to illuminate one side of a sheet of white paper, first with the light falling from a 
measui-ed portion of the zenith, and afterwards with the light from the whole heavens, 
determining in each case, at the other side of the paper, by means of a photometric 
arrangement, the amount of visual illumination. As, however, this method of observation 
can only give satisfactory results when the illumination of one side of the paper is pro- 
portional to the intensity of the rays which fall on the other side with very various angles 
of incidence, it was necessary, fli’st of all, to determine ivhether these conditions were 
sufficiently fulfilled under the circumstances of the experiment. 
The following airangement was made hr order to determine this point. A, fig. 4, 
Plate XLIII. represents a blackened tube closed at one end [a) by the paper about 
to be examined. On a table in front of the tube (A), a large divided quadrant (B) is so 
placed that the centre of the chcle rests on the table directly below the centre of the 
disk of paper at a. In connexion ivith the quacbant is a moveable scale (C) divided 
into millimetres, and upon this scale the standard burner is placed, so that the rays pro- 
ceeding from the flame can be made to fall upon the paper {a) at any given angle of 
incidence. Inside the tube at e is fixed a diaphragm of drawing-paper, the centre of 
which had been rendered transparent by a small piece of stearic acid. When this dia- 
phiagm is illuminated from D by a small constant flame, the stearine spot, as seen 
through the eye-tube {b), appears in a certain position of the lamp {g), to be white on 
a black ground ; if the lamp be now brought nearer, at a certain point the spot dis- 
appeais, and when the flame is brought still nearer, the spot is again seen, but now 
