48 LIGHT IN RELATION TO TREE GROWTH. 
This would be true if the clearness of the atmosphere remained 
always the same. As a matter of fact, when the atmosphere is not 
clear the different rays are not equally absorbed by it, and the result 
is that the measurement of the chemical rays alone does not give a 
correct value for the intensity of all other rays. 
Furthermore, the light which is measured in the forest under the 
shade of the crowns is a “mixed ” light, which consists of diffused and 
direct light passing unchanged through the openings between the 
crowns; of light which is reflected from the leaves, branches, and 
trunks; and, finally, of light which has passed through the foliage. 
By the photochemical method it is possible to measure accurately 
only light which penetrates into the forest through the openings be- 
tween the crowns and foliage, and which is of the same composition 
as the sunlight in the open, except that all the rays composing it are 
equally weakened. If the light in the forest consisted only of this 
kind of light, then it would be possible to compare its intensity with 
that inthe open. Since, however, the light in the forest is a “ mixed ” 
light, which partly consists also of reflected ight and helt trans- 
mitted through the green foliage, such comparison is apt to lead to 
errors. 
Helmholtz has pointed out that landscapes rich in green vegetation 
appear as a rule too dark in photographs. He ascribes this to the 
fact that the rays which act upon the photographic paper are not 
reflected by the green leaves of living vegetation. Roscoe has ex- 
pressed the same idea, and has demonstrated further that “ chem1- 
cal” rays do not pass through green leaves. It is easy to become 
convinced of the truth of this statement by a very simple experiment. 
Tf fresh green leaves are placed on sensitive photographic paper and 
exposed for several minutes to light, the parts covered by the green 
leaves, even very thin leaves, such as those of basswood, remain en- 
tirely or nearly white, while the paper all around them assumes a 
dark color. 
These facts prove that green leaves not only do not reflect the 
chemical rays, but do not even transmit them. Investigations of 
Krause, Engelmann, Reincke, and Timiriazev have shown that fresh 
ereen leaves, as well as the alcoholic solution of such leaves, absorb 
the rays of different wave lengths unequally. Sachs demonstrated 
by direct experiments that rays of different wave lengths penetrate 
plant tissues differently. He found that the ultraviolet rays of the 
solar spectrum were absorbed more fully than other rays, and were 
retained by the most superficial layers of the tissues. The violet, 
blue, and yellow penetrated somewhat deeper into the tissues, while 
the red and partly the green rays penetrated most. In other words, 
the rays which are most active in producing the blackening of the 
