ott] LIVINGSTON—LIGHT INTENSITY AND TRANSPIRATION 425 
that they received perpendicular rays from the direction of the sun 
at the time of observation. 
As to the portion of the entire radiation which is absorbed 
by these various instruments, it is possible to say very little at 
the present time. It is fairly certain that portions of all the 
various wave-lengths of light, as well as of the infra-red and ultra- 
violet, are absorbed by the thermal instruments. On the acti- 
nometers we may be as certain that little effect is produced outside 
the actinic rays. Since the heating effect is the one which we 
are interested in at present, because our inquiry deals with the 
evaporation of water from green leaves, it is obvious that on a-priori 
grounds the thermal instruments are to be regarded as the most 
reliable. The quantitative statement of this whole matter must be 
left to some future time. 
One other theoretical point may be mentioned here with refer- 
ence to the interpretation of the results obtained with these instru- 
ments. It must be borne in mind that other factors besides radia- 
tion intensity are active in the control of evaporation, both from the 
Porous cup and from the plant. Thus, in the night time the evapo- 
ration rate and that of transpiration are by no means mil, while the 
readings of the instruments which do not deal with evaporation 
will vanish at that time. From this fact we may expect the differ- 
ences between two light intensities as shown by the non-evaporating 
instruments to be much greater than those shown by the evaporat- 
ing ones. The latter always record the rate without light influence 
plus that due to light, the former can show no rate without the 
influence of radiant energy. It is possible so to manipulate the 
Porous cup as to obtain readings from it directly comparable to 
those from the black bulb thermometer, but this cannot be entered 
into here. 
Experimentation 
It was my good fortune to be able to spend the summer of 1910 
at the Desert Laboratory, Tucson, Arizona, under auspices of the 
Department of Botanical Research of the Carnegie Institution of 
ashington. During this period, and with the assistance of Dr. 
Witi1am H. Brown, now of the Michigan Agricultural College, a 
number of lines of inquiry which had been previously begun were 
