378 THE AMERICAN MONTHLY [Dec, 



matter as a small Cyclotella ; and, according to the meth- 

 od used, these were given equal weight in the report. 



These facts led us to seek some unit in terms of which 

 we could express, as nearly as possible the actual quan- 

 tity of animal and plant matter present. After a long 

 investigation it was decided to adopt the standard 

 already in use for the estimation of the amorphous mat- 

 ter, namely, a square twenty microns on a side, having 

 an area of four hundred square microns. This, when mod- 

 ified in case of organisms known to be either very thick 

 or very thin, gave us practically a unit of volume. This 

 modified unit of area probably approaches as near to an 

 exact volumetric unit as it will be found practicable to 

 use. 



The area of four hundred square microns was selected 

 as the standard unit because it had already been used in 

 estimating the amorphous matter, and our observers 

 had therefore become accustomed to it; because it was 

 about the size of several of our most common organisms; 

 and because it was a unit whose size could be easily car- 

 ried iu the mind. This unit can be used with but little 

 extra labor. Many organisms are very constant in size ; 

 these may be counted, as heretofore, and reduced to the 

 new standard by multiplying by a previously determined 

 factor. Filamentous forms which are of a constant 

 width may be measured in length and then reduced to 

 standard units. In the case of irregular masses, and of 

 organisms and colonies which vary in size, a special es- 

 timate must be made for each. After a little practice 

 this can be done very quickly and accurately. It will be 

 found of great advantage to have the ocular micrometer 

 divided as follows : — the square, which should cover one 

 square millimeter on the stage of the microscope, is first 

 divided into four equal squares and each of these quar- 

 ters sub-divided into twenty-five smaller squares, each 

 of which is equivalent to twenty-five standard units. 



