34 LIGHT IN RELATION TO TREE GROWTH. 
Miiller (1877) attempted to determine the comparative light re- 
quirements of different species by arranging leaves of the same species 
in layers, one above the other, until no light could penetrate through 
them. This he accomplished by placing the leaves over a small aper- 
ture in a perfectly dark room and by causing the sun rays to pene- 
trate into this dark room through the aperture perpendicularly to 
the surface of the leaves. Light was entirely prevented from enter- 
ing the dark room by a thickness of 10 leaves of ash, 11 of elm and 
basswood, 13 of alder and oak, 15 of aspen and maple, 17 of blue 
beech, and 20 of beech. These figures are indicative of the shade 
which each species can endure, and may therefore serve as a criterion 
of their tolerance. 
PHYSICAL METHODS.’ 
A more direct way of determining the light requirements of forest 
trees is to measure with instruments the hght itself, just as weight is 
measured by means of scales, temperature by means of thermometers, 
air pressure by means of barometers, and air humidity by means of 
psychrometers. 
Sunhght is not homogenous, but is composed of rays of many dif- 
ferent wave lengths. The white sun ray in passing through a prism 
reveals seven different colors, known as the solar spectrum, and these 
in turn are made up of an infinite number of color gradations. The 
different rays composing the solar spectrum produce different effects 
upon matter. Thus, the rays which produce by their action upon 
the retina the sensation of vision are luminous rays and are found 
chiefly in the yellow portion of the spectrum, decreasing toward the 
red and the violet. The rays producing the heating effect are calorific 
rays and are centered in the red portion. They are very low in the 
ultraviolet and high in the infra-red. The rays which affect photo- 
graphic paper or other light-sensitive substances are actinic or 
chemical rays. They are usually greatest in the violet or ultraviolet 
portion of the spectrum. In measuring light, therefore, it is possible 
to measure either the luminous effect or the heating effect or the 
chemical effect. 
Plant life is differently affected by different rays of the solar 
spectrum. In order to determine accurately, therefore, the effect of 
light upon tree life, not merely the luminous effect or the chemical 
effect, but the effect of each separate ray of the spectrum should 
be measured. Unfortunately, up to the present time no accurate and 
practical instrument has been invented by which the effect of the 
different rays of the spectrum can be measured. The methods of 
1 This discussion is based largely on a detailed description of the physical methods 
prepared by G. A. Pearson, in charge of the Coconino Forest Experiment Station, Forest 
Service. 
