STUDIES IN TOLERANCE OF NEW ENGLAND FOREST TREES 11 



percent, and Austrian pine about 60 percent of the total light. In a 

 beech stand which had been lightly thinned the crown absorbed 93 

 percent and a stand of Austrian pine similarly treated absorbed 80 per- 

 cent. 



The measurements here recorded vary so widely in respect to the 

 so-called minimum that it would seem as if the accuracy of the method 

 must be open to question. 



LIGHT READINGS 



- The light values given in this section of the bulletin were made 

 by the writer or his pupils, with the Clements type of photometer. This 

 is a modification of the Bunsen-Roscoe-Wiesner instruments with 

 which it is possible to make 25 separate readings in the field for each 

 instrument. On account of its size and ease of operation it is adapted 

 admirably to studies in the field. The usual method of operation is 

 described by Clements (4) and need not be repeated here. In our work 

 three persons were used in making the standard, one with a stopwatch, 

 the other two with photometers. Exposures of from one to 25 seconds 

 were made. Whenever possible this standard was made at actual 

 noon on June 21 ; if clouds intervened at midday, on the first succeed- 

 ing clear day. In the work done at Camp Bogardus, Douglas Lake, 

 Mich., the standard was made the first clear day after work began. 

 One of the strips of solio paper thus exposed was taken as the stand- 

 ard, glued to cardboard on which was indicated the length of exposure 

 for each of the 25 tints thus obtained, wrapped in black paper and 

 stored in a pasteboard box in the dark room where it was well pro- 

 tected from light and heat and showed little or no change during the 

 season. 



In actual work the readings taken in the field are compared with 

 the standard in order to determine the light intensities. The matching 

 of colors was done usually by one and the same person in order as 

 much as possible to eliminate its personal equation. However, re- 

 peated tests made with several individuals have shown that appreciable 

 difference in the light intensities as thus determined are uncommon. 

 Hence it is safe to assume the essential accuracy of the observation. 

 The light values determined when the standard was made were given 

 the value "1," and all data were figured to this basis. 



It is extremely difficult to get reliable data on the light intensity 

 in the forest because the forest cover is not uniform, the light varies 

 greatly within short periods of time, more particularly because of pass- 



