PERIODICAL LITERATURE 431 



There is serious need for a reinvestigation of tolerance and a more 

 precise definition of its relation to light requirements. As a part of the 

 basis for such work and an understanding of the effect of sunlight 

 on forest vegetation and the difficulties encountered in measuring its 

 effect the article by Professor Pulling on sunlight and its m'easurement 

 is of interest. 



Sunlight applies strictly only to that portion of the sun's radiation 

 that is capable of producing sensations of light in man. We cannot 

 say that only this portion affects forest vegetation. We know, however, 

 that its effect is greater than any other portion of the sun's radiation. 



The first part of the author's paper is a brief, non-technical state- 

 ment of our present knowledge concerning the distribution and the 

 amount of energy in the incoming radiation from the sun. The second 

 part outlines the complex subject dealing with the measurement of 

 the sun's radiation. A short bibliography is added to indicate the 

 locations of some of the more thorough discussions of the important 

 points. Part one is illustrated by many graphs and special emphasis 

 is placed upon the distribution of energy in the sun's spectrum and the 

 distinction between prismatic and normal spectra. 



Little is said or can be said concerning* the total effect of the atmo- 

 sphere upon radiation at any giv6n time. In general, however, losses 

 from incoming solar energy occur in seven ways : (1) General scatter- 

 ing by the permanent gases of the atmosphere; (2) general scattering 

 by water vapor; (3) selective (banded) absorption by the permanent 

 gases; (4) selective (banded) absorption by water vapor; (5) absorp- 

 tion and reflection by clouds; (6) absorption and reflection by dust; 

 (7) absorption in chemical reactions. 



Each of the above conditions increases with the amount of the 

 effective substance between the object receiving the radiations and 

 the sun, also with the total distance traversed by the light beam through 

 the atmosphere with any given concentration of effective material. 



Part two, which deals entirely with the measurement of solar radia- 

 tion, emphasizes the fact that as it is a variable both in quantity and 

 quality at any given station its influence on vegetation can only be de- 

 termined by artificial illuminants or else the incoming radient energy 

 must be constantly measured. A brief note is made of the use of 

 artificial illuminants. 



Each of the three general methods for measuring radiation (radi- 

 ometry. photometr>\ and actinomctry) is briefly discussed as well as 



