376 Biology in America 



microscope, each square covering a known area of the cell 

 at a given magnification. Suppose, for example, the amount 

 of water filtered through the net to have been 1,000,000 cubic 

 centimeters (rougiily 1100 quarts) and tiiat the catch is con- 

 tained in 100 cubic centimeters of fluid. Suppose further 

 that file volume of the cell is one cubic, and its area ten square 

 centimeters, and the area of each one of ten parts counted is 

 1/10 of a square centimeter. If these ten parts contain 100 

 specimens of some species, then the number of individuals 

 of that species in the 1,000,000 cubic centimeters strained by 

 the net would be 100 (number of individuals counted) 

 X [10-^ (10 X 1/10)] (ratio of total cell area to area in 

 which specimens were counted) X 100 -^ 1 (ratio of volume 

 of concentrated catch to volume of cell) = 100,000, or one in 

 every ten cubic centimeters of the water filtered; assuming 

 of course that the organisms are uniformly distributed 

 through the concentrated sample and through the counting 

 cell, an assumption which is only approximately true. The 

 method of counting, like that of collecting, is subject to a 

 large error and the whole method is necessarily a very approxi- 

 mate one. In the case of larger forms, which can readily be 

 seen with the naked eye, such as shrimps, jellyfish and small 

 fish, and which are never very numerous in these quantita- 

 tive collections, the number taken in the entire catch is gen- 

 erally counted. 



For counting the very minute animals and plants, if they 

 are abundant, a more accurate method is the use of the 

 centrifuge, by which all of the organisms may be obtained 

 in the concentrate, if the speed of the centrifuge be sufficiently 

 high and the time of centrifuging long enough. Centrifuges 

 are now made which will run at a speed of 3,000-4,000 revolu- 

 tions per minute and carry 100 cc. of water. In general how- 

 ever the centrifuge method is applicable only for the study 

 of the most minute organisms in small samples of water. 

 For general quantitative studies of the organisms present 

 in the sea it is quite impracticable. 



Yet another method of determining the number of organ- 

 isms present in water, is the Sedgwick-Rafter method, so 

 named from Professor Sedgwick of the Massachusetts Institute 

 of Technology, the Avell-known sanitary biologist, and JMr. 

 Geo. W. Rafter, C. E. This has been employed quite exten- 

 sively for studying the organisms (especially the microscopic 

 plants) present in drinking waters. In this method a given 

 quantity of water, usually about a pint, is filtered through a 

 plug of fine sand about three-fourths of an inch deep, which 

 is held in the funnel-shaped end of a tall, narrow cylinder 



