SEA WATER 



227 



that if a photographic plate was exposed for eighty minutw 

 at a depth of 3,280 feet it was blackened by the light rays : 

 but a plate exposed for 120 minutes at 5,578 feet w^as not 

 affected (see Plate 2). 



A most excellent example of the absorption of light by 

 the sea water is furnished by the famous cave at Capri, 

 known as the Blue Grotto of Capri. Within this cave 

 everything is enveloped in the purest blue light. The 

 explanation is to be found in the fact that the only light 

 that can enter the cave itself has to pass first beneath the 

 water which practically fills its narrow entrance. In its 

 passage through the water much of the red light and some 

 of the yellow and green is absorbed, and the only light 

 that can come once more above the surface of the water 

 to illuminate the interior of the cave is composed to a very 

 large extent of blue rays. The blue colour of the sea also 

 owes its origin to this phenomenon, for the colour of the 

 water is due to the reflection of light upwards from the 

 small particles suspended in the water itself. The light 

 reaching a particle at a given depth is thus reflected up- 

 wards and has to pass once more through the depth of 

 water it has already traversed in its downward journey. 

 Much of the red and yellow light will become absorbed on 

 this upward journey if this has not already happened on 

 its downward passage, and it is mostly blue and green 

 light which can survive to appear above the surface once 

 more and give the sea its typical colour. As all these rays 

 of light are being absorbed in their downward passage it 

 is natural that the actual strength of the light is gradually 

 diminishing the deeper one goes. In the open ocean the 

 strength of light is already too weak at a depth of 100 

 fathoms to support much plant life (see Plate 2), and 

 below this depth few living plankton plants are to be 

 found. Nevertheless this upper layer of water, loc 



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