Extinction and Modification of Light 193 



the blue light becomes completely predominating; this was observed 

 directly by Beebe in his bathysphere dive off Bermuda. An observer 

 looking straight down into the water from the deck of a ship in the 

 clear tropical parts of the ocean sees the blue color resulting from this 

 selective absorption. Since sunlight has been shorn progressively of 

 its longer and shorter wavelengths, the only component remaining to 

 be scattered upward again to the eye is the blue. Anyone who has 

 seen the intense indigo of the tropical oceans will never forget it. 



7000 



8000 



5000 6000 



Wavelength 



Fig. 6.6. The spectral distribution of solar energy at the earth's surface and 



after modification by passage through the indicated meters of pure water. Similar 



hght conditions would be found in the clearest ocean and lake waters. (Clarke 



1939, AAAS Publ. No. 10.) 



The foregoing has shown that even pure water absorbs light at a 

 very rapid rate compared to air and causes a profound change in 

 spectral distribution. In the clearest parts of the ocean and in ex- 

 ceptionally clear lakes the optical properties of the water are closely 

 similar to those of pure water. Other natural waters contain sus- 

 pended particles and dissolved material in sufficient quantities to 

 cause a further reduction in transparency and a further alteration in 

 spectral composition. 



Suspended material in the water includes living organisms that in- 

 crease the extinction of the light and thus modify their own environ- 

 ment in this respect. Illumination is reduced by beds of kelp along 



