tgt1] LIVINGSTON—LIGHT INTENSITY AND TRANSPIRATION 423 
by the absorbed rays. These instruments now make use of some 
form of photo-sensitized paper, and the reading is either the length 
of time required to produce a certain standard shade of color in 
the paper, or the depth of color produced by an exposure of a certain 
length. WresNER’s instrument (see his Lichtgenuss der Pflanzen) 
belongs to this class. They are not photometers in any true sense, 
but really actinometers, measuring only the actinic effect of the 
light. The paper may be modified so as to give sensitiveness in 
any part of the spectrum, but the region to which they are sensitive is 
always rather limited, and the sensitiveness is not uniform through- 
out this region. Another obstacle in their operation comes from 
the difficulty of procuring proper standard colors; a third arises 
from the fact that the comparison of the color produced with the 
standard depends to a great extent upon the observer’s judgment, 
the sensitiveness of his retina, etc. Two forms of actinometer were 
tested in this work. They were exposed to the action of the light 
till the sensitive paper attained the shade of the, standard, the record 
being made in seconds. 
5. The simplest form of actinometer for our estimations is the 
ynne exposure meter, for sale by most dealers in photographic 
goods. It is very convenient to use, the paper therefor is appar- 
ently carefully standardized, and with each package of paper is 
furnished a slip of non-fading standard color suited to that partic- 
ular lot of paper. 
6. The other instrument of this class to which we had recourse 
is the Clements actinometer, a modification of the Wiesner type 
of instrument, using any form of photographic paper which the user 
may wish. It is described by CLEMENTS in his Research methods in 
ecology. As there recommended, I used “‘solio”’ printing out paper, 
and made my own standard color (water colors, afterward var- 
nished), which was no easy task. As will be shown in the records 
of these tests, there is no doubt that this method is as satisfactory 
in operation as that of the Wynne, but the former is somewhat 
More difficult. I have had evidence, moreover, that the “‘solio” 
brand of paper is rather more apt to alter with age (at least in a 
warm climate) than the Wynne paper. 
Since all of these instruments, both thermal and actinic, depend 
