162 FOUNDERS 0¥ OCEANOGRAPHY 



Colour and Light in the Sea. 



Some of the varied colours of the sea can be explained, but 

 we probably do not yet fully understand them all. Pure 

 water has in bulk a clear blue colour, which is an optical 

 effect due to the blue rays of the sun's light being less 

 absorbed than the red rays, and therefore the characteristic 

 colour of the open ocean, where there is no disturbing influ- 

 ence, is blue. Variations in the tint of blue and the occurrence 

 of other colours, such as green, yellow, and grey, are due to 

 impurities in the water or minute organisms present in great 

 quantity. Green and yellow tints of different intensities 

 occur near land, the olive-green of the Antarctic is caused 

 by enormous quantities of Diatoms suspended in the water, 

 and the deep blue round coral reefs is said to be due to car- 

 bonate of lime in solution. Other local or temporary con- 

 ditions may affect the colour profoundly — for example, a 

 plague of minute Dinoflagellates (Gonyaulax^ etc.) may dis- 

 colour the sea red for miles. 



The light rays from the sun penetrate the sea to varying 

 depths, according to their nature and the clearness of the 

 water, the red rays being absorbed first and the blue pene- 

 trating more deeply. The effects of the light upon a photo- 

 graphic plate have been traced down to 300 fathoms off 

 Capri, in the Mediterranean ; and, in the Atlantic, Helland- 

 Hansen's light-recording apparatus showed light-rays 

 affecting the plate on an exposure of eighty minutes at 

 1,000 metres (547 fms.), but at 1,700 metres the plates were 

 not affected after an exposure of two hours. Sir John 

 Murray therefore considers the " photic zone " to be in 

 general the upper 500 fathoms in the open sea. Near land 

 and in oceanic water containing impurities or many minute 

 organisms the light penetrates to lesser depths. 



The degree of penetration of the light -rays has a profound 

 effect upon the plants and animals of the sea. Green Algae, 

 which are only found near the surface, assimilate their neces- 



