
1905. | On Vegetable Assimilation and Respiration. 451 
may be as much as three or four times that of the sun alone, and that too, in 
summer. 
As regards red rays, the noon-total-daylight curve for the year agrees very 
closely with that calculated for red rays in clear sun alone, so that the clouds 
appear to reflect upon the earth just about as much red light as they stop out 
of the direct sunlight. This would tend to discount the influence of 
sunlight in a process such as assimilation. 
As regards green constituent rays it is different, and the clouds in general 
reflect much more than they stop directly out of the sun’s rays. A few 
observations made concurrently on the “chemical intensity ” of the action of 
total daylight at noon on sensitised paper seemed to show that the actinic 
rays are proportional to the red, and about 25 times as intense (in ratio 
that is, to the relative abundance of these two kinds of rays in a normal 
candle). 
The whole trend of these observations is to increase our @ priori estimate 
of the assimilatory value of diffuse light as compared with direct sunlight. 
This also has been the outcome of the measurements recorded in this paper. 
The highest assimilatory value of diffuse light recorded is 0°0192 gramme CO, 
in Experiment IX, and this is equal to all the assimilation that is possible at 
about 27° C. with Helianthus, or at 34° C. (31° initially) with cherry-laurel. 
The intensity of the blue rays will not always serve as a measure of the 
intensity of the red rays, particularly so when the sun is low in the heavens. 
Abney* has measured the brightness of the various parts of the solar 
spectrum with different altitudes of the sun. The smaller the altitude, the 
longer is the path that the sun’s rays travel through the atmosphere, and as the 
air is more absorbent for blue rays, so these tend to die out more and more as 
the sun sinks and the light becomes redder. We may quote the following 
data :—T 
Sun’s altitude............ 90°-0 30°-0 14°3 (ies. about 0° 
Atmospheric mass ...... 1:0 2°0 4:0 80 32°0 
Red. A, A =-0°76 ....:. 0°95 0-91 0°81 0°66 0-107 
Orange, D,X¥ = 0°59... 0°87 0°75 Oop 0:32 0-001 
Blue, F, > = 0°49...\... 0:74 0-54 0°30 0:09 0-000 
Total sun brightness... 0°84 0°70 0°50 0°21 0002 
In this connection it is interesting to recall how late in the evening 
assimilation can be detected. The rays active in photography die oat rapidly 
towards sunset, but Experiments [I, III, and IV show that the red rays 
* Abney, ‘ Phil. Trans.,’ 1887 and 1893. 
t Hann, ‘ Lehrbuch der Meteorologie,’ Leipzig, 1901, p. 13. 
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