204 PRINCIPLES OF STRATIGRAPHY 



OPTICS OF THE WATER. 



Of the greatest importance to organisms living in the sea or in 

 fresh water is the depth to which sunHght penetrates. It is to this 

 depth only that chlorophyll-generating plant life occurs, which in 

 turn determines the depth to which animal Hfe subsisting on it may 

 penetrate. Thus the Characea exist in Lake Geneva to depths of 

 25 m., while Hypnum lehmanic has been found as deep as 60 m. (a 

 single case.) Diatoms extend in summer to a depth of 20 m., in 

 winter to 80 m. 



The transparency of water varies greatly. Thus in tropical seas 

 the animal and plant life can easily be seen at a depth of 20 meters 

 or more, while 45 meters has been recorded near the Philippine 

 Islands as a depth at which corals were still visible. A white plate 

 submerged by von Kotzebu in tropical waters of the North Pacific 

 was still visible at 50 meters, while Wilkes found that the depth at 

 which such an object became invisible varied in the same region 

 from 31 to 59 meters. Observations on tiie Gaselle showed that 

 the visibility depth of other colors was less than that of white, the 

 depth at which yellow disappeared being 88% of that at which white 

 disappeared, while for red it was yy% and for green 67%. Ex- 

 periments with submerged lights gave for the Lake of Geneva the 

 following results : 



Depth 

 visible. 



1. Moderator lamp with vegetable oil (July 18, 1884) ..41.3 m. 



2. Edison lamp, y candle power (]\Iarch 15, 1886) . . . .50.9 m. 



3. Arc light (August 8, 1885) 88.0 m. 



A small electric lamp of 8 candle power was submerged in the 

 Black Sea, where the depth was over 2,000 meters. The depth at 

 which the point of light disappeared varied from T^.y m. to 40 m., 

 that at which the light disappeared entirely varied from 43 to 

 yy m., according to the locality. Measurements made by expos- 

 ing photographic plates at various depths showed that the influence 

 of the light still extended to 500 or 550 m. depth. From various 

 measurements it appears, however, that in the sea only the upper 

 300 meters receive light throughout the day, while at 350 meters 

 depth light is present for only 8 hours. 



In fresh water the depths beyond which photographic plates are 

 no longer affected appear to be much less. Thus in Lake Geneva 

 F. A. Forel found that silver chloride plates were aflfected in sum- 



