78 A TEXTBOOK OF OCEANOGRAPHY 



were made in the following proportions : two of yellow to 98 

 of the blue (2), five of the yellow to 95 blue (5), and so on. 

 Thèse solutions were enclosed in glass tubes of i centimètre 

 diameter, and then arranged like the rungs of a ladder. The 

 comparisons take place in the absence of direct sunlight or of 

 light reflected from the heavens. Up to the présent not many 

 colour investigations with Forel's scale hâve been made in the 

 open océans. The greatest surface of mid-ocean shows a blue 

 colour, G to 2 on Forel's scale, especially in tropical and sub- 

 tropical régions. Green colours are met with near the coast 

 and in shallow water generally, and again in Polar waters. 

 The North Sea in its northern and central portions is dark 

 blue. The purest and deepest blue is that of the Sargasso Sea, 

 although the South Atlantic, Indian, and Pacific Océans are 

 not very différent. 



Small quantities of sea-water are colourless both by reflected 

 and transmitted light. Greater depths appear blue by trans- 

 mitted light, according to the expérience of divers. In depths 

 of 13J to 16 fathoms a dark red animal appears black to a diver ; 

 on the contrary, the green or bluish-green algœ appear paler 

 than usual. This is because sea-water absorbs the red rays 

 much more strongly and rapidly than the green. 



The absorption of light in sea-water plays an important 

 rôle in the life-history of marine animais and plants. It has 

 been estimated that at a depth of 96! fathoms yellow light has 

 the intensity of the light of the full moon on the surface of 

 the land. 



The power of assimilation of marine plants varies con- 

 siderably. For instance, the Rhodophyceae assimilate nearly 

 two and a half times as fast in the blue rays of the spectrum 

 as in the yellow. Consequently algae of this group are found 

 flourishing in greater depths than others. The conditions 

 under which the algse of the plankton, with their yellowish or 

 brownish corpuscles, assimilate is not thoroughly understood. 

 One species (Halosphœra viridis) is, however, met with living 

 in depths of over 1,000 fathoms, both in the Mediterranean 



