colours disappears more quickly than that of light. There are in the 

 retina two sorts of sensitive cells: the first, the cones, distinguish 

 colours, but are less sensitive to light; the others, the rods, are more 

 sensitive to light, but do not perceive colours. 



A very simple experiment will demonstrate this. Let us go into a 

 dark room lighted only by a feeble blue lamp : the bulb appears blue, 

 but if we glance aside, the rest of the room does not have any appearance 

 of colour. The same phenomenon is observed in photography: the 

 black and white films are more sensitive than the colour films. This 

 explains why I cannot say what is, in fact, the colour of the last luminous 

 ray. The proverb 'every cat in the twilight's grey' applies literally in 

 such a case: the last glimmer seems grey. However, Schiller's diver 

 speaks of a purple darkness and Beebe of a violet light. Only a photo- 

 graph of the spectrum would permit an answer to this question. One 

 thing is certain in any case : i8 inches of water already absorbs a great 

 part of the red light and a few yards intercepts it altogether. To prove 

 this, just look at the water in a white-enamelled bath-tub : it appears 

 bluish because, already, the water in it absorbs a part of the red light. 



Let us not forget that the permeability of sea water to light rays is 

 not constant : it varies according to the place and from one day to the 

 next. One cannot be exact; all that can be said is that below some 

 hundreds of yards the darkness is complete. 



COLOURS OF SUBMARINE VEGETATION AND FAUNA 



I hope I may be permitted to digress into the domain of biology. 



On land we admire the colours of plants and animals. But why is the 

 finery of the peacock so beautiful ? Simply to arouse the interest of the 

 pea-hen. Butterflies, too, recognize their kind by their colours and the 

 designs that adorn their wings. Lastly, what of the flowers, which 

 unfurl chromatic marvels to attract the insects which fertilize them ? 



What is the origin of these colours ? Darwin provided an answer : 

 from generation to generation the handsomest peacock has had the 

 greatest success, or in other words, has had the most descendants. He 

 it is who has had the greatest number of opportunities of transmitting 

 his colours to posterity. It has always been so : from century to century 

 selection has enriched his characteristics. Among the butterflies the 

 handsomest, too, are those most certain to be reproduced. In the same 

 way the most beautiful flowers have necessarily produced the greatest 

 number of seeds. 



["7] 



