CONDITIONS OF EXISTENCE 29 



rapids. They make a shapeless mass without it. A few animals 

 require very complete aeration or they die very quickly. Suckers 

 appear to die from lack of oxygen while the rainbow darter adds 

 something to the water in which it lives which is not removed by 

 artificial aeration and which kills the fish unless the number of fishes 

 is small or the water changed often. 



Light penetrates clear water to great depths. During the cruise 

 of the Michel Sars the penetration of sufficient light to markedly 

 affect the most sensitive photographic plates in 80 min. was found 

 at a depth of 1000 meters (latitude 31° 20', June 5-6. Sun nearly 

 over head; for methods see Murray and Hjort). No effect was 

 obtained at 1700 meters with an exposure of two hours. Light 

 sufficient to affect the plates in 2 hours lies somewhere between 

 1000 and 1700 meters. There were many rays of all kinds at 100 

 meters but least of the red. Though penetration is rarely as great 

 in fresh water as in the sea, light may possibly penetrate to the 

 bottom of Lake Baikal which is the deepest fresh water lake known 

 (1300 to 1700 meters are reported). 



In temperate latitudes light does not penetrate so far vertically 

 because it enters the water obliquely. The depth of penetration 

 can easily be calculated for any latitude or season from the angle 

 of declination of the sun, when the penetration in similar water is 

 known for other latitudes and seasons. 



The most important factor limiting the penetration of light into 

 fresh water is turbidity. Forel found the light penetration in 

 Lake Geneva (Switzerland) greatest when the lake contained least 

 sediment. Table II gives the depth of light penetration in Lake 

 Geneva in March when it is clearest. Forel used much less sen- 

 sitive plates than were used on the Michel Sars, the sun was much 

 lower in the horizon and the locality 15 degrees farther north. 

 Thus Forel's records show that light did not diminish notably in 

 the first 25 meters, fell off gradually in the second 25 meters and 

 then dropped off rapidly to zero for his plates at no meters. Fol 

 and Sarasin with more sensitive silver salts than were used by Forel 

 found that light reached 200 meters in winter. It is altogether 

 probable that the plates and apparatus of the Michel Sars would 

 show much light at three or four times the depth given by Forel. 



