The Sea-water and its Physical and Chemical Properties 



55 



undergoes a progressive change in direction since the most obUque light is diminished 

 most while the diffuse light formed by scattering increases continuously. 



The first light measurements on the open sea were made by Poole and Aitkins 

 (1924). Detailed measurements have been made more recently by Clarke (1933, 1936, 

 1938) and by Clarke and Oster (1935); (see also Utterback 1936). For an example 

 Figs. 29 and 30 show the percentage reduction in intensity of light in different parts 

 of the spectrum for the surface layers of the Sargasso Sea and of the Gulf of Maine. 



Percentage of surface light 

 OOI 005 0-1 05 1-0 50 10 50 100 



20 

 40 

 60 

 80 

 ^ iOO 

 120 

 140 

 160 

 180 



Q 



Fig. 29. Decrease in the intensity of light in the Sargasso Sea for different spectral ranges as 

 a percentage of the intensity at the surface (according to Oster and Clarke). 



OOI 



Percentoqe of surface light 

 005 0-1 05 10 5 10 



50 100 



Fig. 30. Decrease in the intensity of light in the Gulf of Maine for different spectral ranges, 

 as a percentage of the surface intensity (according to Oster and Clarke). 



As a striking feature the extinction curve is almost linear with depth so that within 

 the spectral region investigated the extinction coefficient is almost constant and is 

 independent of the depth. The violet and the blue are most strongly affected by the 

 turbidity, the red is least affected. 



The extinction coefficient for shelf and coastal water is considerably larger than for 

 ocean water, approximately two to three times larger or even more. Its size represents 

 only the order of magnitude of the coefficient since these types of water show large 

 variations both in time and locality. Swedish light measurements, which have been 



