BESEAECH 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, 

 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. Spectra 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- 

 con ling 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 

 t<» 10.000 Angstrom units. Such plates, of course, are short-lived, 

 being \crv sensitive to heat. 



I n 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 sped rographic observations, to use several plates, cover 

 ing the cut ire range of the spectrum with as great thoroughness as 



Me. 



