CONDITIONS OF EXISTENCE 31 



in some of the western mountains, are very clear and one can see 

 to depths of 5 to 15 meters. Depth at which objects may be seen 

 is measured by lowering a white disc 20 cm. in diameter known as 

 the disc of Secchi. 



When light penetrates water the red rays are most rapidly ab- 

 sorbed, then orange, yellow, etc. In the Michel Sars measure- 

 ments there were scarcely any red rays at 500 meters, one-half the 

 depth at which light was measured. Fol found off Nice that when 

 down in 30 meters of water he could see a stone 7-8 meters away and 

 a bright object at a distance of 25 meters. Red animals looked 

 black, while green and blue green algae looked quite bright. 



In water there is no dawn or twilight. The surface of the water 

 reflects practically all the light when the rays come to it very 

 obliquely. Fol found that in 10 meters of water solar light dis- 

 appeared quite suddenly long before sunset. In Funchal Harbor 

 (Madeira) the Prince of Monaco used Regnard's apparatus in which 

 a film is moved before an opening by clockwork, and found that at 

 20 meters in March the day lasted 9 hours whereas at 40 meters 

 the film showed the effects of light for only about 15 minutes at 

 2 P.M. 



Light profoundly influences the migrations and distribution of 

 animals probably largely because it has a marked effect on life 

 processes. Unfortunately, however, with the exception of ultra- 

 violet light which penetrates the atmosphere into low altitudes in 

 minimal amount, very little is known of the actual physiological 

 effects of light. Under experimental conditions animals usually 

 avoid or select the blue end of the spectrum. Red usually acts as 

 darkness or very faint light. Thus animals living in very strong 

 light usually accumulate in blue or violet when exposed to spectrum 

 colors. Animals living in darkness collect in the red. Animals 

 living in moderate light usually wander about throughout the spec- 

 trum but a majority congregate in the blue. Probably animals 

 are affected through photo-chemical reactions which are brought 

 about most often by the blue end of the spectrum. Daphnias 

 select the brightest part of the spectrum which is the green or the 

 yellow for most organisms, brightness being determined by some 

 specific effect of particular wave lengths upon the light recipient 



