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 light was distinctly seen at a depth 
of thirty-three metres. A few centimetres more, and the clear 
image disappeared. It was replaced by a diffuse light, faintly 
perceptible at sixty-seven metres. 
Messrs. Sarasin and Soret noticed a very characteristie 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 little 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 Бе, 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 light, 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 tropies, 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 
