THE LIFE OF DEVJUS LAKE 21 



to obtain closely accurate results. An essential condition of this 

 method is, of course, uniformity in the plate or film employed. By 

 using the same kind in all experiments, practical uniformity of re- 

 sults is probably assured. 



The objections to the third method have been detailed by Shel- 

 ford and Gail (1. c) and may be summarized by the statement that 

 the sensitivity varies in different cells and under different condi- 

 tions (temperature, time of illumination and quality of light). 



The pyrlimuometer of Birge and Juday measures the radiatior 

 (heat) energy of light, rather than its chemical effect, which latter 

 is the more important factor to be determined, — since the heat effect 

 can be found by thormometric methods. 



The objections to the use of a photo-electric cell would appear 

 to be its expense, difficulty of construction and operation, especially 

 on rough waters and the absorption of some rays by the glass cell. 



In all of these methods probably the greatest difficulty in- 

 volved is in the variable condition of the water surface and of the 

 air at different times. "When the surface is rippled there is ob- 

 viously much greater reflection than when it is smooth. The differ- 

 ence in penetration of the light thru the surface layer may vary 

 momentarily more than 100%. Thus Shelf ord and Gail (1. c. pp. 

 160-1) have shown that on Puget Sound the light penetrating the 

 surface varied in three minutes from 67.3% of the total (in the 

 air) to 26.0% with changes in the surface, due to tide ripples. Even 

 marked changes in intensity of light, as recorded by photo-electric 

 cells, may occur within brief intervals in bright sunlight. "Marked 

 difference in apparent brightness of the sunlight and in the reading 

 may take place in five or ten minutes when no clouds are visible." 

 (Shelf ord and Gail, 1. c. p. 155). 



The results obtained by these various investigators naturally 

 vary widely according to the various methods and conditions of the 

 experiments. No two waters are exactly alike in character, nor does 

 any one body remain constant at all times. In general, the penetra- 

 tion is greater in winter than in summer, partly because of the 

 direct influence of temperature on the absorptive power of water, 

 partly because of the amount of plancton and amorphous matter 

 present at these seasons ; tho individual waters will vary greatly 

 at different times and under different conditions. 



Helland-Hansen*, using photographic plates, found in the At- 

 lantic Ocean a penetration of from 900 to 1500 m., while Grein 

 (1913)**, employing a similar method, found a penetration of about 

 1500 m. Andersen and Walker (1920), in their work on the Sand- 

 hill lakes of Nebraska, found an elimination of light at 4 dm. vary- 



*See Murray & Hjort (1912, p. 257). 

 **Fide' Shelford & Gail (1. c). 



