THE EXPERIENCES OF A DIVER. 221 



There is an angle at which the proportion of rays reflected to rays 

 refracted becomes so much against the latter that the illumina- 

 tion of the bottom falls off very abruptly. 



The transparency of the water along the littoral varies enor- 

 mously. In times of rain, it is clouded by swollen streams pour- 

 ing into it ; in dry and still weather it becomes nearly as clear as 

 in the open sea. There are also capricious and sudden changes 

 caused by currents from the land or from the open sea, which are 

 capable of producing great effects in a few hours. Experiments 

 on the penetration of light, to have any value, should be made 

 very far out. 



When the water is comparatively clear, it still absorbs so much 

 light that at thirty metres' depth, if the sky is clouded, one can 

 not see distinctly enough to collect small animals. In a horizon- 

 tal direction one can not distinguish a rock more than seven or 

 eight metres off. When the sun is shining and the water is very 

 clear, we can see a bright object at twenty or even perhaps at 

 twenty-five metres. But in usual conditions we have to content 

 ourselves with half these numbers. These facts, verified many 

 times in the descents which I have executed with the diving appa- 

 ratus of my laboratory at Nice during the last three years, appear 

 to me important from several points of view. 



It is evident that a submarine boat can not see its way under 

 these conditions. Slow as may be its movement, there will not be 

 time for it to retreat if it sees some obstacle rising in front of it ; 

 for it would not be more than ten metres away from the impedi- 

 ment at the moment of perceiving it. It will always have to take 

 its directions before going down, and to sail only upon a ground 

 the relief of which has been carefully explored. Submarine navi- 

 gation will thus always be confined to limits which the genius of 

 man since it can not change the transparency of water will 

 never be able to enlarge. 



These observations are also of great interest from a biological 

 point of view. We can see every day that agile marine animals 

 living in the illuminated strata of the waters fishes, lobsters, 

 and cephalopods are in the habit, when they are frightened, of 

 giving themselves up to a very rapid flight and quickly stopping. 

 They feel that a few metres are enough to put them out of the 

 range of vision of their pursuer. Some even take the pains to add 

 to the obscurity of the water by discharging their ink, as the 

 squids do, or stirring up the mud, after the manner of many 

 fishes. Marine animals may well be near-sighted; for of what 

 use to them is a long vision when they can at most see only a few 

 metres away ? Hence their crystalline lens is bulged into a 

 nearly spherical shape. They live in a world of surprises, and, as 

 it were, in a perpetual fog. The nets we stretch for them would 



