40 LIGHT IN RELATION TO TREE GROWTH. 
of measure is taken the shade made by an exposure of one second at 
meridian at a given day and place when the sun is not obscured by 
clouds. 
Since the standard is not “ fixed,” it should therefore always be 
kept in a dark, cool place. The exposures are made by quickly draw- 
ing back the slide, thus exposing a section of the sensitive paper. 
The paper is usually exposed until a medium shade is obtained, be- 
cause very light or very dark shades are difficult to match with the 
standard. After each exposure the disk B is turned to the next 
number. When the entire strip has been exposed, it is removed, 
labeled, and placed in an envelope or light-proof box, where it is 
kept until it is compared with the standard. 
At the end of the day’s field work the exposed strips are compared 
with the standards, in the absence of chemically active rays. The 
standard is laid upon the table and fastened at each end with pins 
to prevent curling. The exposure to be compared is passed along 
the standard until the shade is found with which it matches. The 
relative light intensity is then found from the time ratio between 
the particular shade in the standard and the exposure with which it 
matches. Thus, if the tint has been obtained by an exposure of 60 
seconds in the forest, while the shade of the standard which it 
matches has been obtained by an exposure of only 3 seconds in 
the open, then the light intensity in the forest is 3/60, or 1/20, or 
0.05 of that of meridional sunlight in the open. The difficulty with 
Clements’ method, even more than Wiesner’s, lies in matching the 
tints obtained in the forest with those of the standard; yet with 
some practice fairly accurate results are obtained by means of Clem- 
ents’ photometer. 
ABSOLUTE AND RELATIVE LIGHT INTENSITY. 
Since plants seldom make use of the total daylight but only of a 
portion of it, it is of greater practical value to know how much of 
the total daylight a given species actually requires for its growth 
than simply to know the light intensity in the crown expressed in 
absolute figures without relation to the light intensity in the open. 
The ratio between the light intensity which a given plant actually 
enjoys and the hght intensity in the open is the “ relative” lght 
intensity. The light intensity in the open, expressed in Bunsen- 
Roscoe units, is the “ absolute ” ight intensity. Thus, if the hght in 
the open has a chemical intensity equal to 0.80, Bunsen-Roscoe units, 
while at the same time the light intensity in a tree crown is only 
0.20, then the relative hght intensity, or the amount of light the tree 
actually receives, is 20/80, or one-fourth of the total daylight. 
