252 



DEPTHS OF THE OCEAN 



CHAP. 



is therefore difficult to compare the plates 

 it may at least be maintained that there 



with no filter. It 

 quantitatively, but 



must be many blue rays, though hardly any red ones, at a 

 depth of 500 metres. Series of experiments with and without 

 filters were also made at a depth of 100 metres ; in forty minutes 

 all the plates were over-exposed, those with a red filter only a 

 little, those with a blue one very much, so that there are many 

 rays of all kinds at 100 metres, though fewest of the red. When 

 plates without colour-filters were exposed on the top and on 

 the sides of the cube simultaneously, the plate on the top proved 

 to be more strongly influenced than the others. This fact is 

 not without interest, as it shows that the rays in the clear 

 tropical waters have a distinct direction at 500 metres, not 

 having yet become altogether diffuse ; shadows should, then, 

 be thrown even at that depth. 



Regnard constructed an apparatus for determining the length 

 of the day at different depths, in which a clockwork arrange- 

 ment inside a cylinder causes a photographic film to pass before 

 an aperture. At the end of March 1889 the Prince of Monaco 

 made some experiments with Regnard's apparatus in the harbour 

 at Funchal, Madeira ; the water was not so clear as in the open 

 sea, so the times recorded may be rather short. At 20 metres 

 the day lasted eleven hours ; at 30 metres it began at 8.30 a.m. 

 and ended at 1.30 p.m., the sky becoming overcast ; at 40 metres, 

 with the sun shining brightly, the film exhibited only a slight 

 influence of light for a quarter of an hour about 2 p.m. These 

 and a few other experiments show that the day becomes 

 gradually shorter, and the intensity of light decreases, as the 

 depth increases. 



The Swiss naturalist, Hermann Fol, has several times gone 

 down in diving dress off Nice to examine the bottom. At a 

 depth of 10 metres the solar light disappeared quite suddenly in 

 the afternoon a long time before sunset. At 30 metres the 

 light was so bad that it was difficult to gather the animals on 

 the bottom ; he could see a stone only at a distance of 7 or 8 

 metres, whereas shining objects in favourable positions could 

 be discerned at a distance of 25 metres. He also noticed that 

 dark red animals (like Muriccea placornus) looked quite black, 

 while the green and green-blue algse appeared lighter in colour. 

 This is explained by the fact that the red light disappears much 

 sooner than the blue. A coloured object will always look black 

 when untouched by rays of its own colour. As the white sun- 

 light contains all colours, objects display in it their proper tint, 



