1897J MICROS TOPICAL JOURNAL 119 



water, etc., are all disturbing elements attecting their 

 growtli. 



In order to determine the effect of light upon their 

 growth it was found necessary to make experiments in 

 the open reservoirs under conditions practically the same 

 as those found in nature. 



The method employed was an extremely simple one. 

 It consisted of suspending bottles filled with water from 

 the same source at different depths in the pond, the bot- 

 tles being tied to a rope which hung from an anchored 

 buoy. After a certain time the bottles were drawn to 

 the surface and the water examined, records being kept 

 of the number of diatoms in each sample })efore and after 

 exposure. The bottles vuried in capacity from 150 to 

 1,000 cc. lu the first five experiments they were tightly 

 stoppered, but in the later ones silk bolting cloth was 

 tied over the mouths of the bottles, and inverted glass 

 tumblers were placed above. The latter arrangement 

 gavf much heavier growths on account of providing bet- 

 ter opportunity foi- the cii-culation of air and for the re- 

 newal of food supply. 



Without describing the (experiments of [Forel Forel, 

 F. A. "Le Leman, monographic limnologique," Laus- 

 anne, 1895] and others upon the intensity of light at 

 various depths, it may be said that the decrease in the 

 intensity below the surface is due to two causes — ab- 

 sorption by the water, and tlie pr^-sence of fine particles 

 which act as a screen. The reduction of light in passing 

 through water is supposed to follow the law that as the 

 depth increases arithmetically the intensity of the light 

 decreases geometrically. For example, if the intensity 

 of the light falling upon the surface of a pond is repre- 

 sented by I, and if ^ of the light is absorbed by the first 

 foot of water, then the intensity of light at the depth of 

 one foot will be J; the second foot of water will ab- 

 sorb i of 'i, and the intensiiy at a depth of two 



