RESEARCH METHODS IN STUDY OF FOREST ENVIRONMENT. 57 
are certainly suggestive of the need for spectroscopic measurements, 
if we are to determine with any degree of accuracy the kind of light 
available in the forest. 
It is self-evident that the simplest means for examining into the 
absorption of various wave lengths by leaves is to examine the spectra 
of beams of light which have passed through individual broad leaves 
or layers of needle leaves, charting the marked bands of absorption r 
and comparing such charts with similar ones for uninterrupted sun- 
light. Likewise the spectrum of the diffuse light in the forest may be 
examined, while the spots of direct light which have reached the 
ground through large or small openings may be expected to show 
essentially the same character as light in the open. 
Such observations, while doubtless of great value in gaining an 
insight into the difference between species, and representing the first 
work which one would naturally undertake in spectroscopy, have 
only limited value because of the difficulty in reducing the absorption 
evidence to quantitative terms. There would naturally be also a large 
subjective error. 
10. Spectro photographs. — Photographs of the various spectra 
which may be examined in forest studies obviously have an advantage 
over mere observations in their permanency, and over drawings in 
their completeness. According to Baly (52), ordinary photographic 
dry plates are fairly sensitive to rays within the lengths 2,200 to 5,000 
Angstrom units, or from about the limit of the blue well into the 
ultra-violet. In the " orthochromatic " plates and films of commerce 
the tendency toward very rapid action in the ultra-violet region is 
suppressed by the use of dyes, so that the shades and tones of the 
visible spectrum are more clearly brought out. 
Plates approaching monochromatic value have been prepared for 
several regions, the principle being in all cases to stain the plate with 
a dye which absorbs strongly the rays it is desired to bring out. Thus 
a red- colored dye may be used to bring out yellow and green. Ac- 
cording to Baly (52) again, Abney succeeded in preparing a photo- 
graphic emulsion which was sensitive in the infra-red to 20,000 
Angstrom units, and the solar spectrum was actually photographed 
to 10,000 Angstrom units. Such plates, of course, are short-lived, 
being very sensitive to heat. 
In addition, there are, more recently, so-called panchromatic plates, 
which have a very wide range of sensitiveness. 
Until more is known as to the part which the infra-red rays play 
in the chemical activities of the plant, it would seem to be the part 
of wisdom, in spectrographic observations, to use several plates, cover 
ing the entire range of the spectrum with as great thoroughness as 
possible. 
