128 PHYSICAL FACTORS: LIGHT 



paper is not sensitive to the longer wave lengths of light, the ones 

 most used in photosynthesis. For making comparisons between 

 different habitats, however, the method has considerable value. 



The Clements photometer is the instrument that is most used in 

 light measurements by the silver chloride method. This consists 

 of a circular metal box around which a strip of "printing out" 

 paper may be fastened. The "cover" is slipped on outside of the 

 paper and fastened at the center by a thumb-screw in such a way 

 that it may be revolved. The cover has an opening at one side which 

 is closed by a shutter. When an exposure is to be made the shutter is 

 withdrawn for a definite length of time as measured by a stop-watch 

 and the cover is then revolved one unit-space in preparation for the 

 next exposure. Twenty-five exposures may be made with one strip 

 of paper. The paper is later removed from the instrument in diffuse 

 light and compared to a strip of standard tints. A recent model of 

 this instrument is of vest-pocket size and has a stop-watch attached. 

 All the operations are automatically made by pressing a plunger. 



The Weston Sunlightmeter is an instrument which in some respects 

 is more convenient for field work than the photometer. It consists 

 of a special photo-electric cell with a sensitive ammeter calibrated 

 directly in foot-candles so that it is only necessary to expose the 

 instrument to the light and read the light values in foot-candles on 

 the instrument scale. The range covered by this instrument is 

 approximately that of the visible spectrum and, since a large number 

 of readings can be made in a short time the instrument is very useful 

 for comparing various habitats. Full light in the open may be used 

 as a standard and the light in other habitats may be expressed as 

 percentages of the standard. 



REFERENCES 



Adams, J. : The Effect on Certain Plants of Altering the Daily Period of Light, 



Ann. Bot., 37, 75-94, 1923. 

 Some Further Experiments on the Relation of Light to Growth, Am. 



Jour. Bot., 12, 398-412, 1925. 

 Atkins, W. R. G., and Poole, H. H. : Photoelectric Measurements of Illumina- 

 tion in Relation to Plant Distribution, Part I, Sci. Proc. Roy. Dublin Soc, 



18, 277-298, 1926. 

 Braid, K. W.: The Measurement of Light for Ecological Purposes, Jour. 



Ecology, 11, 49-63, 1923. 

 Burkholder, p. R.: The Role of Light in the Life of Plants, Bot. Rev., 2, 



1-52, 1936. 

 Burns, George P. : Measurement of Solar Radiant Energy in Plant Habitats, 



Ecology, 4, 189-195, fig. 1-3, 1923. 

 Evans, M. W., and Allard, H. A.: Relation of Length of Day to Growth of 



Timothy, Jour. Agric. Res., 48, 571-586, 1934. 



