450 Dr. F. F. Blackman and Miss G. L. C. Matthaei. [Apr. 11, 
Here we note that sky-light increases its ratio to sunlight as the altitude 
decreases, being just half at 45°. 
For a horizontal surface on the earth Brennand oaleuleties from the above 
data :— 
Sun’s altitude. Sunlight. Diffuse light. Total light. 
sa) 0-012 0-051 0-063 
45 0-069 0-083 0152 
60 0-140 0-089 . 0229 
These numbers show how large a part diffuse light plays in the total 
illumination when the sun is low. 
Roscoe* has made a large number of measurements of “ chemical intensity ” 
of radiation by the photographic method worked out by Bunsen and himself 
in 1862. He finds that the “ chemical action” (ze., that on sensitised silver 
paper) of the sun depends only on its altitude independently of the hour of 
day or the latitude. 
The presence of clouds causes large departures from the values calculated or 
found for cloudless skies; the “clouds act as mighty reflectors of light” and 
“the presence of a thin film of cloud may enormously increase the total 
chemical intensity.” 
The rays that act on silver salts do not, however, play the chief part in 
assimilation. Objective measurements of the intensity of action of the less 
refrangible rays of natural light have not, so far, been carried out. Possibly, 
the activity of the living chloroplast may be yet used for this purpose. 
Measurements of the intensity of total daylight on a horizontal plate have 
been made daily at noon for some years by L. Weber.t He measured the 
intensity of the red rays (about 7X = 630) and the green rays (about X=541) 
of the total daylight, comparing them photometrically by eye with the same 
rays in a standard artificial ight. He finds that the ratio of red to green 
varies in the daylight independently of its intensity. 
The highest value observed in three years for the total daylight at noon was 
on July 5, viz., 154,300, and the minimum on December 12, viz., 655, the total 
mean of the three years being 36,185 metre candles. 
The amount of the total daylight on any given day that is attributable to 
diffuse sky-light was arrived at by deducting the calculated brightness of the 
sun’s direct light at noon on that day. In this way it is shown that on some 
days, with the suitable arrangement of clouds, even at noon the total light 
* Roscoe, ‘ Phil. Trans.,’ 1863, 1865, 1867, 1870; and ‘ Roy. Soc. Proc.,’ vol. 15, 1866. 
+ L. Weber, ‘Meteorol. Zeitschrift, vol. 2, 1885; and ‘Schriften Naturw. Vereins, 
Schleswig-Holstein,’ vol. 10, 1895. 
