76 A TEXTBOOK OF OCEANOGRAPHY 



green only 67 per cent., of similar white discs. In the Irish 

 Sea the maximum visibility is about 12 fathoms. As a rule 

 the visibility is greatest at sunrise and sunset, and decreases 

 with the height of the sun. 



A second method of determining the transparency of sea- 

 water is to sink an electric light on a dark night, observe the 

 depth at which the lamp itself disappears, and also the depth 

 at which the diffused light is no longer visible. This has been 

 done by Spindler and Wrangell in the Black Sea in localities 

 where the depth was well over a thousand fathoms. The 

 greatest depth at which the lamp (of 8 candle-power) was visible 

 was a little over 21 fathoms, whereas diffuse light was still 

 noticeable at 35 J fathoms. 



Still another method is the photographic. Sensitive plates 

 are exposed at varying depths until after long exposure they 

 cease to darken. By this method the presence of light 

 has been detected, off Capri, at depths of 270 to 300 

 fathoms. 



More recently an improved photometer has been devised 

 by Helland-Hansen, and used in the Norwegian investigations 

 on the Michael Sars. A camera of special construction is 

 lowered to the required depth while hermetically sealed. A 

 messenger runs down the line and detaches the bottom part of 

 the camera in which the plates are exposed. After the required 

 interval a second messenger runs down the line which, in 

 effect, puts the lid on the camera, which can then be drawn up 

 to the surface. 



Highly sensitive panchromatic plates are used, and with 

 Wratten and Wainwright's -three-colour filters (red, green, 

 and blue) it is possible, when required, to exclude certain 

 portions of the spectrum. 



Helland-Hansen 's experiments show that a plate exposed 

 for 80 minutes at i ? ooo metres (546 fathoms) was blackened by 

 rays of light, but an exposure for 120 minutes at 1,700 metres 

 (940 fathoms) produced no result. It follows that the limit to 

 which light penetrates lies somewhere between these limits. 



