306 THREE CRUISES OF THE " BLAKE." 



the water of the lake, and the depth to which light penetrates. 

 Three candles in a lantern — the flame being fed by a contin- 

 uous current of air — were visible at a depth of thirty metres, 

 in pure water. An electric hght was distinctly seen at a depth 

 of thirty-three metres. A few centimetres more, and the clear 

 image disajDpeared. It was replaced by a diffuse light, faintly 

 perceptible at sixty-seven metres. 



Messrs. Sarasin and Soret noticed a very characteristic absorp- 

 tion ray (in the red near B) in the spectrum light which had 

 traversed a certain layer of water. They further observed that 

 the distance of clear vision varied very httle with the increase 

 of the brilliancy of the luminous body and its absolute dimen- 

 sions. Photographic experiments in the deep portions of the 

 lake showed the effect of light on the sensitive plates down to 

 two hundred and fifty metres. This depth seems to be, at least 

 for the plates now in use, the extreme limit of action of the 

 sun's light. " Below this point the lake is a vast dark cham- 

 ber." 



In March, 1885, Messrs. Fol and Sarasin concluded, from 

 their experiments at Villefranche, that in fine weather the last 

 rays of light were dissipated at a depth of about four hundred 

 metres below the surface of the sea. 



Nowhere can the effect of hght, heat, and motion be bet- 

 ter realized than on any beach in the tropics, or upon a coral 

 reef. There, exposed to the full glare of a tropical sun, a pro- 

 fusion of animal life flourishes, unknown in more temperate 

 latitudes. We meet its parallel again in the arctic regions, 

 where an immense number of specimens developed during the 

 long arctic days replace the diversity of forms of tropical 

 realms. The varying conditions of rocky coasts, of long sandy 

 stretches, of mud flats, of gravelly beaches, — whether ex- 

 posed to or sheltered from the action of the sea, — whether 

 situated in deep or shallow water, in the tropics, the temper- 

 ate, or the polar regions, — give us an almost endless variety 

 of physical conditions under which our marine fauna and flora 

 flourish, in striking contrast with the conditions in the abyssal 

 and intermediate districts of the oceanic basins. 



Pelagic animals, living as they probably do within a belt 150 



