28 PHYSICAL PROPERTIES OF SEA WATER 



showed that photographic plates were blackened at a very great depth. 

 A plate exposed for 40 minutes at a depth of 500 m showed strong black- 

 ening, another exposed for 80 minutes at 1000 m was also blackened, 

 but a third plate which was exposed for 120 minutes at 1700 m showed no 

 effect whatever. These experiments were made at noon on June 6, 1910, 

 with a clear sky. At 500 m it was found that the radiation had a distinct 

 downward direction, because plates exposed at the top of a cube were 

 much more strongly blackened than those exposed on the sides. 



In other experiments colored filters were used, which showed that 

 the red portion of the spectrum was rapidly absorbed, whereas the green 

 and blue rays penetrated to much greater depths. Quantitative results 

 as to the absorption at different wave lengths were obtained by using 

 spectrophotometers, but the methods were laborious and not sensitive 

 enough to be used to great depths. 



The introduction in recent years of photoelectric cells has made 

 possible rapid and accurate determinations of extinction coefficients in 

 various parts of the spectrum. A number of different instruments have 

 been and still are in use, but a standardized technique has been proposed 

 by a committee of the International Council for the Exploration of the 

 Sea. Because of the wide variation in absorption at different wave 

 lengths, efforts have been directed toward measuring exactly the absorp- 

 tion in narrow spectral bands. The determinations are accomplished 

 by lowering stepwise a photoelectric or photronic cell enclosed in a water- 

 tight container and provided with suitable color filters, and by observ- 

 ing on deck the photoelectric current by means of a sensitive galvanometer 

 or a suitable bridge circuit. The measurements must be made at con- 

 stant incident light either on clear, sunny days or on days when the sky 

 is uniformly overcast, because the rapid variations in incident light that 

 occur on days with scattered clouds will naturally lead to erroneous 

 results as to the absorption. If one wishes to determine the percentage 

 amount of radiation that reaches a certain depth, it is necessary to make 

 simultaneous readings of the incident radiation on board ship. 



These methods give information as to the absorption in la3''ers of 

 definite thickness. Instruments for measurements of the transparency 

 of sea water at given depths and of the scattering of light have been 

 designed by H. Pettersson and have been used for determining relative 

 values. It has been demonstrated, particularly, that at boundary sur- 

 faces sharp variations in transparency and scattering occur. Since the 

 study of the absorption of radiation in the sea is in rapid progress, several 

 of the following generalizations are presented with reservations. 



The main results as to the character of the extinction coefficient of 

 radiation of different wave lengths in the sea can be well illustrated by 

 means of data that Utterback, and Jorgensen and Utterback have pub- 

 lished. Utterback has attempted to determine the extinction coefficient 



