406 Dr. F. F. Blackman and Miss G. L. C. Matthaei. [Apr. 11, 
general diffuse light, a wooden tube of somewhat bigger section than the leaf, 
and about four feet long, was fitted tightly on to the front of the bath so as to 
cut out practically all the diffuse light. An oblique observation-hole cut near 
the base of the tube allowed the illumination of the leaf to be inspected. For 
exposure to diffuse light alone, while the sun was shining, a thin board, about 
18 inches square, fixed obliquely at the end ofa 20-foot batten, was supported 
high up in the air so as to cast a shadow on the leaf. This hindered the 
access of only a small portion of the diffuse sky light, but had, of course, to 
be carefully watched and shifted by hand that a shadow might always be 
kept on the leaf. 
For certain critical experiments it became essential to have known fractions 
of the sun’s luminous radiation falling upon the leaf. An account of the 
method adopted to ensure this will be found in Section V. 
Section Il.—On the Internal Temperature of Leaves Exposed to Natural 
Illumination. 
In a previous paper* we have shown that the radiation from a high-pressure 
incandescent gas-light of nominal 1000 candle-power, causes a considerable 
rise in the internal temperature of leaves exposed close to it even after the 
radiation has passed through a thick stratum of water. 
It becomes, therefore, important to determine what heating effect natural 
illumination, and, in particular, full normal insolation will produce. 
The greater part of the dark-heat radiation from a source of light would be 
absorbed by interposed water and glass, but a further amount of radiation, not 
utilisable for the chemical work of assimilation, is absorbed by the more 
opaque leaf aud some excess temperature of the leaf over its environment 
is inevitable with powerful light. 
With leaves exposed to bright light in a slow current of damp air there is 
not very much cooling of the leaf by evaporation and the internal leaf 
temperature is a factor that cannot be ignored. 
In the present section we shall consider, in some detail, a few cases which 
illustrate the temperatures that can be attained inside leaves exposed to 
various degrees of natural illumination, on the one hand when the leaf is in 
the open air, and on the other hand when it is enclosed in a glass chamber, 
with or without water-cooling. The temperatures are in all cases determined 
thermo-electrically.t 
* “ Assim. and Resp. III,” p. 75. 
+ A crystalline transparent alum-plate would cut off somewhat more heat, but this is 
an impracticable expedient, while alum solution is rather less efficient than water, ef. 
Shelford Bidwell, ‘ Brit. Assoc. Reports,’ 1886, p. 309. 
t For details of procedure, see “ Assim. and Resp. III,” p. 76. 
