
1905.] On the Physiological Processes of Green Leaves. 39 
The most satisfactory way of meeting this difficulty is to employ the method 
of rotating sectors which has been largely used by Abney in his investigations 
on colour-measurement. For this purpose the Cambridge Scientific Instrument 
Company constructed for us an apparatus consisting of a series of moveable 
metal sectors, which could be adjusted on a revolving axis in such a manner 
as to cut off from an object placed under the sectors any desired amount of 
the total solar radiation falling on it. 
The motive power was, in our case, supplied by a small water turbine, but 
the use of a small electric motor would much simplify the construction. The 
broken disc, formed by the sectors, could be inclined at any desired angle, 
and when rotated in front of a leaf, placed parallel to it, cut off a perfectly 
definite proportion of the solar radiation, and the effect could be compared 
with that produced by the full radiation falling on another leaf alongside the 
first. 
Measurement of the Intensity of Radiation. 
The intensity of radiation falling on the leaf-surface was determined by 
means of a Callendar’s radiometer, consisting of a pair of differential platinum 
thermometers, the one black and the other bright, wound on flat plates of 
mica and placed side by side in a flat rectangular glazed case, which was 
mounted on an adjustable stand so that the radiometer could be placed 
alongside the leaf and in exactly the same plane. 
The platinum coils occupy an area of about 75 sq. cm., and the difference 
in temperature between them, which is proportional to the intensity of the 
vertical component of the radiation, is determined by connecting them with a 
Callendar’s recorder, consisting essentially of a “ Wheatstone’s bridge or 
potentiometer in which the movement of the slider along the bridge-wire is 
automatically effected by relays, worked by the current passing through the 
galvanometer between the bridge-arms.”* 
A simple form of planimeter attached to the instrument integrates the 
curve drawn by the pen on the revolving drum, and from the known 
constants of the instrument the planimeter-readings can be readily converted 
into water-gramme-units (calories) per square centimetre per minute, due 
regard being paid to any extra resistances which it may be necessary to insert 
during the course of the experiment for the purpose of keeping the pen 
within the range of the drum. 
We are much indebted to Professor Callendar for his assistance and advice 
* Professor Callendar described his recorder in detail in ‘ Engineering’ for May 26, 1899. For 
a generai description of the Callendar’s radiometer, see ‘ British Association Report’ for 1898, 
p. 796. 
