History of Forest Ecology 413 



Early in the 19th century trees were recognized as being either 

 tolerant or intolerant of shade and there were several gradations 

 between. This classification had been obtained in a purely 

 empirical manner, and while it does not pretend to be accurate, 

 it has great practical value and is generally used even today. 

 It was not, however, until about 50 years later that the scientific 

 determination of light values was first attempted. 



The first attempt to measure light rays was made by Bunsen 

 and Roscoe about 1862 when they introduced the use of photo- 

 graphic paper for determining light intensities in climatological 

 investigations. Theodore Hartig was one of the first to attempt 

 to determine quantitatively the light requirements of trees by 

 this means. His experiments were made about 1877. Reinke in 

 1884 to 1885, by means of an instrument which he called the 

 Spectrophore, broke up light by means of a prism and directed 

 certain colored rays upon plants to determine the effect. Cieslar 

 did much work upon the role of light in the forest, and Wiesner 

 has measured light and determined the minimum intensities in 

 which both tolerant and intolerant species could endure. These 

 two men have done most of their work in the last 25 years. 



It was with the hope of giving mathematical expression to light 

 requirements of trees that led Wiesner about 1896 to devise his 

 insolator. This instrument makes use of the well known law, 

 formulated by Bimsen and Roscoe, that products of light intensity 

 and time of exposure correspond to darkenings of silver chloride 

 paper of like sensitiveness. This method is the common one in 

 use today. The instrument, however, which is most used in this 

 country for this kind of work is the Clements Photometer, de- 

 vised in 1905 upon the same principles as the Wiesner instrument. 



The most serious objection usually raised against the use of 

 this method is that it measures the light rays which produce 

 chemical changes on photographic paper and not those that are 

 of importance in the chlorophyll apparatus. It is well known that 

 photographic paper is affected mainly by the blue to violet rays 

 of the spectriun and that chlorophyll utilizes mainly the red rays. 

 This is not an unsurmountable objection, since it is well known 

 that the intensity of the red and yellow rays of the spectrum may 

 be proportional to the intensity of the blue to violet rays. In 

 other words, the chemical rays are proportional to the photo- 

 synthetic rays and, therefore, their effects are proportional, so 



