OPTICS OF THE WATER 205 



mer to depths of 45 m. only after exposure for a full day, while in 

 winter the depth increased to no meters. Lake Constance (Boden- 

 see) gave a depth for the same phenomena of 30 m. in summer and 

 less than 50 m. in winter. Sensitive plates coated with silver bro- 

 mide and exposed for a whole day in the Lake of Zurich were 

 afifected to a depth of 100 meters in August, 1881, and in the 

 Walensee to a depth of 140 m. in October, 1891. In the Lake of 

 Geneva after 10 minutes" exposure Fol and Sarasin found in August 

 that the plates were faintly affected at. 113 m. and not at all at 237 

 m. In September very faint results were shown at 170 m. and in 

 March, 1885, at 192 m., while at 235 m. no efifect was obtained. 

 In the next year, however (March, 1886), slight results were ob- 

 tained at 240 m. From these facts Forel concludes that the limit 

 of light efifect for silver bromide ig between 200 and 240 meters, 

 varying with the season and the water body, while at the same 

 time the depth varies for different substances sensitive to light. 



The recent work in the North Atlantic by the Michael Sars has 

 brought out some very interesting results. A series of measure- 

 ments of the intensity of the light at various depths was made, by 

 a photometer carrying panchromatic plates and gelatine color filters. 

 Near the Azores the light strongly affected the plates at 100 meters' 

 depth, the red rays being weakest and the blue and violet strongest. 

 At 500 meters the blue and violet rays still made distinct impres- 

 sions, the violet and ultra violet still affecting the plate at 1,000 

 meters. At 1,700 meters, however, not the faintest trace of light 

 efifect occurred after 2 hours' exposure. Observations in several 

 latitudes showed equal intensity of light as follows : 



In 33° N. lat. at about 800 meters depth. 

 In 50° N. lat. at about 500 meters depth. 

 In 67° N. lat. at about 200 meters depth. 



These depths correspond to those which were found to be the 

 upper limit of the red pelagic crustaceans (Acanthephyra), as well 

 as that of certain black pelagic fish (Gastrostomus, Cyema) in the 

 same latitudes, so that these organisms are found only where during 

 the daytime the chemically efifective rays from the violet portion 

 of the spectrum are alone active, or at depths where the red forms 

 are as invisible as the black ones. It is only at night that they 

 rise into the upper strata of the sea. (lIjort-15; Murray and 

 Hjort-24: 557.) 



