438 Dr. F. F. Blackman and Miss G. L. C. Matthaei. [Apr. 11, 
diminution of light, because it is in both cases limited by the temperature to 
the maximal value. 
As the sky then began to cloud up it was impossible to employ the 
fractioning screen, and a reading was therefore taken of the effect of diffuse 
light only, removing the sun-tube, and putting up the shadow-board. With 
this illumination we got an assimilation of 0°0136 at 29°1 C., which is a trifle 
below the standard maximum for the temperature, 00143, which may well be 
due to the decline after three hours’ maximal assimilation at this fairly high 
temperature. 
These values are plotted with crosses in fig. 6, where the lower dotted line 
represents the curve of assimilation falling off with the time-factor from the 
hypothetical initial value of 0°0167, through the standard value of 0:0148 
(after about two hours), to the observed value of 0°0136 after three and a half 
hours. 
Apart from the time factor it is obvious that all three kinds of illumination 
—diffuse light alone, direct sun alone, and direct sun plus diffuse light—are 
producing practically the same amount of assimilation, not, of course, because 
they are equal in intensity, but because the assimilation is limited by the 
temperature, all the lights being in excess. | 
Having by this experiment got full evidence as to the magnitude of the 
assimilation maximum at 29°5 C., the next experiment was planned for a 
fraction of the sunlight. A piece of gauze was employed that transmitted 
0°62 of the whole incident light. 
In Experiment XIV the leaf was set up at 9.15; the day was cloudless, 
but with a thin haze. As the haze obstinately remained, the chamber was 
darkened for two hours. Then at 11.25, the haze being nearly gone, the 
wooden tube was put on with the 0°62 screen, and this kept pointed to the 
sun. An estimation at noon gave 0:0154, just the theoretical value as shown 
in fig. 6, and maximal for the leaf-temperature, showing that even 0°62 sunlight 
is more than sufficient for maximal assimilation at 29° 5 C. 
Heavy. clouds came up soon after this reading, and the experiment was 
abandoned, and the next one was made with a piece of perforated zinc with 
much smaller holes, and transmitting 0:28 sunlight. 
In Experiment XV a leaf was set up by 9 a.M.; the day was brilliantly 
sunny, with large, but very remote, cumuli drifting slowly across the sky. 
At 10.14, after a long cloudless spell, it seemed that an estimation might be 
started, but this had to be interrupted in the middle for the passage of one 
big cloud across the sun. At the moment of eclipse the current was shifted 
by hand back into the preliminary tube, and kept there for 20 minutes after 
the cloud had passed, and then it was returned to the estimation tube. At 
