896 PEOI^SSOE BUNSEN AND DE. H. E. EOSCOE’S PHOTO -CHEMICAL EESEAECHES. 
sunshine must be allowed to faU during the experiment, either upon the opening of the 
tube (fig. 6), or upon the hollow metallic cap (fig. 7). This is best prevented by holding 
a round sheet of pasteboard fixed on a pole between the sun and the photometer, at a 
distance of about 3 metres from the latter, so that the amount of difiPuse light cut off is 
inappreciable. 
As the relative amount of illumination of the sky is materially altered by the presence 
of the smallest cloud, it is only when the heavens are perfectly clear that any general 
conclusion can be drawn from these observations. Perfectly cloudless days are, however, 
so uncommon in our latitudes, that on an average not more than six or eight occur at 
Heidelberg in a year*. We must hence consider it fortunate that we were able to 
complete a series of hourly observations from sunrise to sunset upon a day during the 
whole of which the blue sky was not obstructed by the smallest particle of cloud. 
A series of these observations was made on June 6th, 1858, at the summit of the 
Geisberg, a hill in the neighbourhood of Heidelberg, the height of which is 3 / 6 metres 
above the sea, and 106 metres above the river Neckar. On the highest point of the 
wooded hill, a platform, of about 40 feet in height, had been built, rising above the tops 
of the trees ; and on this platform, which commanded a perfectly free horizon on every 
side, the visual illumination of the heavens was determined. During the course of the 
observations, which lasted from sunrise to near sunset, a strong breeze blew from the 
eastward, and the air was so clear, that the Haardt Mountains, situated at a distance of 
twenty miles, were seen not only in distinct contour, but in relief. 
The amount of the illumination of the sky was determined in the following manner : 
the hollow cap was first fixed on the photometer, shaded from direct sunlight by the 
paper screen, and the divided circle so placed that the diaphragm-ring disappeared ; the 
time was then noted and the hollow cap quickly removed. The tube (fig. 6, Plate XLIII.^ 
was next fixed on the photometer, and the divided circle attached to the tube mo'sed 
until the diaphragm-ring disappeared, when the time was again obser'\ ed; the mean of 
these two observations was taken as the time of the experiment. The opening (\}/) of 
the divided circle, measured in degrees and tenths of degrees, when multiplied by 8 58, 
gives, as we have already shown, the amount of light (I) which the diaphragm-ring 
receives from the whole sky, when the unit amount of light is represented bj the amount 
which falls upon the same diaphragm-ring during the same time from a portion of 
zenith-sky equal to y^^th part of the total visible heavens. 
The following Table gives the mean of four independent series of observations made 
on the 6th of June, 1858. The first vertical column contains the numbers of the 
observations ; the second the times of the experiment reduced to true solar time ; the 
third the values of ; and lastly, the fourth contains the required values of I calcrdated 
from the values of 
* According to observations of Padre Secchi, extending over a period of eight years, not more than 
twenty-one cloudless days occur in Eoine in the course of a year. 
