CORALS AND CORAL REEFS — VAUGHAN. 203 



thriving colonies have resulted, but the more central piers bear few 

 or no corals. Light is the only factor I have been able to imagine 

 to be the cause of this result, for the wat^r under the middle of the 

 wharf is of the same temperature as that outside and the food supply 

 is the same both under and outside the wharf. 

 Dana says: 



The range of temperature 85° to 74° F. gives sufficient heat for the develop- 

 ment of the gi-eater part of coral reef species ; and yet the temperature at the 

 100-foot plane in the middle Pacific is mostly above 74°. The chief cause of 

 limitation in depth is the diminished light, as pointed out by Prof. T. Fuchs.^ 



Hjort says in his article on "The Michael Sars JS'orth Atlantic 

 Deep-Sea Expedition : " ^ 



* * * Now, if we calculate the depth to which the rays of the sun pene- 

 trate, after passing through the same distance in the water, assuming always 

 that the rays are direct, and that the rate of absorption is the same, we find 

 that the rays will have passed through the same distance to reach a depth of 

 500 meters in 50° north latitude that they will pass through to reach 650 

 meters in 33° north latitude, or 300 meters in 67° north latitude. 



However, the transparency of the water varies greatly iu different regions. 

 If we take the results of previous observations during different expeditions, 

 we may set down the visible depth in the open sea as being, roughly, 50 meters 

 in 33° north latitude, 40 meters in 50° north latitude, and 25 meters at the 

 outside in the Norwegian Sea in 67° north latitude. Taking this into considera- 

 tion, we find that there will be the same intensity from the rectilinear rays — • 



In 33° north latitude, at about 800 meters' depth. 



In 50° north latitude, at about 500 meters' depth. 



In 67° north latitude, at about 200 meters' depth. * * * 



During the Atlantic cruise of the Michael Sars we undertook a series of 

 measurements of the intensity of light with a photometer constructed by 

 Doctor Helland-Hansen ; to determine the intensity of the different color rays. 

 Doctor Helland-Hansen made use of panchromatic plates and gelatine color- 

 filters. The observation south and west of the Azores (that is to say, at the 

 southern stations) showed that the rays of light strongly affected the plate at 

 a depth of 100 meters. The red rays were weakest here, while the blue and 

 ultra-violet rays were strongest. At a depth of 500 meters the blue and ultra- 

 violet rays were still distinctly visible, and at a depth of 1,000 meters the 

 ultra-violet rays were yet perceptible. In 1,700 meters, however, there was 

 not the faintest trace of light, even after the plates had been exposed for two 

 hours in broad daylight. 



The observations recorded in the foregoing quotation show a dis- 

 tinct decrease in the intensity of the red rays of light at a depth of 

 100 meters. As the maximum development of the deep-sea fauna off 

 the Hawaiian Islands is between depths of 183 and 732 meters and 

 at temperatures between 10° and 4.5° C, depth, temperature, and 

 intensity of light are correlatives. The deep-sea fauna mostly lives 

 at depths too great for the penetration of the red rays, but, where 



1 Corals and foral islands, 3d ed., p. 118; see also Vaughan, U. S. Nat. Mus. Bull. 59, 

 p. 46, 1907. 



" Geographical Journ., vol. 37. pp. 505-50G. 1911. 



