METHODS OF COLLECTING AND PHOTOGRAPHING 83 



then counted by transferring the sample to a glass cell under the 

 microscope. If the bottom of the cell is ruled in squares the 

 contents of a certain number of these may be counted without 

 the use of the eyepiece micrometer and the whole number present 

 in the cell estimated. In the case of the larger and rarer organisms 

 it is best to count all that the cell contains. 



Since the total volume of water from which the catch was made 

 is known, the number of each sort of organism per cubic meter of 

 water or under each square meter of surface may be easily calcu- 

 lated, or the numbers in the entire lake may be approximately 

 determined. 



F. Quantitative Study of the N anno plankton* 

 The nannoplankton may be studied in two ways, namely, by 

 enumerating the various organisms, or by obtaining a sufhcient 

 quantity to determine its dry weight. In the former method the 

 organisms may be counted directly, which is very desirable for 

 the more abundant forms, or they may be concentrated either by 

 filtering or by centrifuging. The filters that are most generally 

 used for concentration are hard surface filter paper and sand. 

 When filter paper is used the filtered organisms are carefully 

 washed from the paper, the volume of the wash water containing 

 the organisms is taken, and samples of it are then used for enumera- 

 tion. It is necessary to use hard surface filter paper in order to 

 prevent undue loss of organisms in the meshes of the paper. Even 

 with the best quality of hard surface paper, many individuals become 

 embedded in the meshes so firmly that they cannot be washed out. 

 For all counting the Sedgwick-Rafter counting cell is to be used. 



The Sedgwick-Rafter sand filter as described by Whipple has 

 been used extensively in sanitary work. In this method also there 

 is a considerable loss of organisms since some of them are so small 

 that they pass between the grains of sand and since it is practically 

 impossible to separate all of the organisms from the sand after 

 filtration. In all filtering methods the filters soon become clogged, 

 which decreases the rapidity of the filtering very markedly. 



* This section has been prepared by Chancey Juday of the Wisconsin Geological 

 and Natural History Survey. 



