20 THE LIFE OF DEVILS LAKE 



of these consists of a white disk, Avhich is lowered in the water and 

 the depths of disappearance and of reappearance upon raising not- 

 ed, the mean of which gives a reading which maj'' be compared with 

 similar readings on other waters, or on the same water at other 

 times, or under different conditions. A modification of this type 

 of apparatus is the turbidity rod of the U. S. Geological Survey, in 

 which a platinum wire is substituted for the disk of the Secchi ap- 

 paratus. In another modification an electric light replaces the disk. 

 Another type depends on the exposure of a sensitive plate or film at 

 various depths for various intervals, the relative degrees of penetra- 

 tion varying inversely as the times of exposure. This method has 

 been employed by several investigators in both marine and fresh 

 water work, using many different kinds of apparatus for this 

 purpose. A third plan is that of Regnard who used the resistance of 

 a selenium cell as registered by a galvanometer to determine the 

 strength of light at different depths in the harbor of Monaco.* 

 Birge and Juday, in their work on Wisconsin lakes and elsewhere, 

 have employed an instrument, the pj^rlimnometer, which measures 

 the radiation effect of the sun at various depths, by means of 

 electrical thermo-couples and a galvanometer. Finally Shelford and 

 Gail (1922) have employed a photo-electric cell consisting of potas- 

 sium mounted in an atmosphere of hydrogen contained in a glass 

 cell, which is connected in circuit with a battery of 80 to 160 volts, 

 and a galvanometer, by means of insulated wire. The amount of 

 current passed thru the cell is proportional to the intensity of the 

 light striking it, and the cell is most sensitive to the blue rays. 



A detailed discussion of the relative merits of these various 

 devices need not be given here. Suffice it to say briefly that all 

 give values which are relative only. 



The first, since it involves a personal factor, is valuable only 

 when used by the same observor at all times. It furthermore tells 

 nothing as to amount or quality of light penetrating to various 

 depths. 



The second method involves the elimination of certain rays by 

 absorption in passing thru the glass window covering the sensitive 

 film. The amount and quality of the rays absorbed by the glass 

 can be tested spectroscopicall}^, however, and in any event is proba- 

 bly small. 



A more serious ob.jectiou to this method is the difference iu 

 relative density with different times of exposure, and development.* 



Another serious objection is that, in comparing the density of 

 plates or films of different exposures with any standard, or Avith 

 each other, the personal error enters and it is difficult, or impossible. 



♦Fide' SliPlford nud Gail (1922). I Iiave not had access to Regnard's original 

 work. 



*Sec Nutting (1912) 



