1905.] On Vegetable Assimilation and Respiration. 449 
temperature-acceleration coefficients) that leaves differ specifically one from 
another. | 
It follows that there is no optumum of light-intensity for assimilation, even 
for specific leaves, still less generally. 
Section VII—The LInmitation of Assimilation by the Natural Environment. 
The relation of the respective intensities of sunlight, diffuse sky-light, 
and total light become of interest in connection with our estimations, and 
with the general question of the available illumination for “sun plants” and 
“shade plants.” The relation of these intensities has been calculated for 
a cloudless sky by Clausius.* The calculations are based on a summing up 
of the various orders of reflections of that part of the sunlight which is 
scattered in passing through the atmosphere. 
Assuming the light that would reach a horizontal surface on the earth 
from a vertical sun to be unity, if the atmosphere were absolutely clear and 
did not scatter any of it, Clausius calculated that with the sun at an 
altitude of 60°, the direct sunlight would be 0°621 and the diffuse light 0°176, 
while at an altitude of 30°, the former would be 0°281 and the latter 0°138, 
and at an altitude of 10°, the former 0:033 and the latter 0:067. 
Thus, as the sun sinks the ratio of the diffuse light to the sun-light 
increases, being about 1 to 2 at 30°. 
Several observers have made direct observations on these points, using the 
darkening of photographic sensitised papers as a measure of light intensity. 
This only gives a measure of the more refrangible rays, and is generally 
spoken of as the “ chemical intensity ” of the radiation. 
Brennandt made measurements in the cloudless sky of India and exposed 
his paper (1) at right angles to the sun’s rays alone in a dark chamber : 
(2) in shadow of a stick to sky alone; and (3) to sun and sky together. 
With the sun below the altitude of 13° the whole diffuse light is more 
active than direct sunlight alone. Above 13° he found the following values 
as means of a large number of observations :— 
Sun’s altitude. Sunlight. Sky-light. Total light. 
We 0:0377 0:0376 00782 
30 01070 0:0628 0°1698 
45 0°1429 00700 0°2128 
60 0°1620 00727 0°2347 
90§ 01751 0:0743 0°2404 
* Clausius, ‘Poggendorff Annalen,’ vol. 72, 1847. 
t Brennand, ‘ Roy. Soc. Proc.,’ vol. 49, 1891. 
{ Roscoe found the altitude for equality to be 19°. 
§ The values for 90° were obtained by calculation. 
VOL. LXXVI.—B. 20H 
