436 Dr. F. F. Blackman and Miss G. L. C. Matthaei. [Apr. 11, 
used to cut off the diffuse sky-light from the leaf. Different specimens of 
commercial wire gauze and perforated zinc were employed, and the area of 
their perforations and the distances of these apart were measured with a 
microscope. The total percentage of opening was thus obtained and the 
percentage of sunlight transmitted would be the same. At a distance of 
4 feet these screens, the holes in which were less than 3 mm. in diameter, 
cast a perfectly uniform field of light. Since the sun’s disk subtends a 
sensible angle (0° 32’) at the earth’s surface, a diverging cone of rays will be 
transmitted through each perforation. These cones soon interpenetrate and 
give a more and more uniformly lighted field the further the illuminated 
object is from the screen. With perforations 3 mm. in diameter the fully 
insolated points die out at 33 cm. from the screen, and at less than four times 
this distance no lack of uniformity of illumination can be detected by 
the eye. 
For the fractions obtained to be of any precise significance, the initial 
sunlight must be of approximately the same intensity in the various cases, so 
that only unclouded weather within the hours close to the middle of the day 
is available. We have consequently had to make a number of unsuccessful 
attempts before the required data could be collected. In some cases a 
reading has had to be interrupted in the middle while a single cloud drifted 
up across the sun, and has been taken up again when the cloud has gone, 
allowing, of course, sufficient interval for the passing away of the effect of the 
momentarily diminished assimilation on the COgs-content of the current. 
When a reading has been obtained in this way it will be stated. 
We propose to determine first what fraction of full sunlight must be 
incident upon a leaf of cherry-laurel to enable it to carry out its maximal 
assimilation at 29°°5. 
Experiment XIII, August 9.—The chamber was in the first instance faced 
to the sun, and received fnll intense sunlight together with the diffuse light 
of a cloudless sky. In the first reading, 9.30 to 10 a.M., the assimilation is 
00152 gramme per hour, at an averaye leaf-temperature of 29°77 C. The 
temperature of the leaf was taken a number of times during this and 
subsequent readings, and was always kept adjusted close to the average 
given, by altering the temperature of the water circulation. With this 
intense illumination the assimilation must be maximal, and the value is just 
above the standard maximum for 29° 7, 72.2, 0°0148, due to this being an 
early reading. 
For the second reading the wooden tube is put on to the chamber, so that 
the leaf receives only direct sunshine without the general diffuse light. The 
assimilation is, however, not lowered (but, as it happens, a trifle larger) by the 
