easy to imagine how quickly I picked my torch up again ! Fortunately 

 the metallic case of my pocket torch had only short-circuited one or 

 two of the twenty-four cells in the battery. But it was an anxious 

 moment. If the battery had been entirely short-circuited, our ballast 

 would have been released and we should have risen to the surface 

 without having reached the desired depth. 



EXTERNAL LIGHTING 



We wanted to observe the submarine world. We had therefore to 

 be able to make use of an external system of lighting as powerful as 

 possible, particularly so when we wanted to take photographs. How 

 should we arrange the projector ? 



We must not forget that sea water is never perfectly transparent. 

 We speak of water being ' perfectly transparent' when visibility extends 

 to 60 yards. 



Let us leave the sea for a moment, and imagine ourselves in a car in 

 a thick fog. In the daytime we can just distinguish the outline of a 

 house 60 yards away. Then night comes on, and when we switch on the 

 headlamps, the range of visibility is reduced to a few yards. We 

 then come into a town. Here the street lamps are placed on each side of 

 the street. If we put out our headlamps our range of visibility increases 

 at once and we can see the street a good distance ahead. We can draw an 

 important conclusion : the floodlights of our bathyscaphe must not be 

 placed inside the cabin : they must be suspended from the float in such 

 a way that the flood of light will cross our visual field in a narrow cone 

 making an angle with our sight-line that as nearly as possible approaches 

 90°. Thus all objects entering the lighted zone will appear to us to be 

 shining against a dark background. We thus have on a large scale what 

 in the laboratory is called ultra-microscopic lighting. 



Our arrangement presents another advantage which is not negligible : 

 if an object appears in the lighted zone, we know at once at about what 

 distance it is and this allows us to estimate its real dimensions. 



Each projector is equipped with a 1000- watt bulb which, very much 

 overloaded, can in a short time develop a lighting intensity of nearly 

 60,000 lumens. 



I shall not spend much time explaining in detail the construction of 

 the projectors. Suflice it to say that they were furnished with incan- 

 descent bulbs specially made by Philips and protected from external 

 pressure by strong steel cases. In order not to reach prohibitive 



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