300 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



The magnification required for Diatoms and other small 

 algse is about 100. The amount of the fluid and the 

 largeness of the magnification offer difficulties in the way 

 of a successful counting. The amount of the fluid is so 

 great that all the Diatoms in a given field of view will 

 not be in focus together, so that it is found convenient to 

 count the diatoms dry. The water is therefore evaporated 

 by the plate being placed in the rays of the sun, or by 

 being placed on the top of a paraffin bath, or any such 

 suitable warm place. 



On account of the mixing of the organisms and the 

 diluted fluid not being quite uniform, different enumera- 

 tions for the same species give different results from 

 different plates, so that it becomes necessary to con- 

 sider how many enumerations are requisite in order 

 to give an average from which the total number 

 of individuals in the whole catch may be accurately 

 deduced. In general, it may be said that, for the 

 species which occur most abundantly, when a fraction, 

 say one-tenth of the square root has been counted it will 

 be found sufficient to aiford a basis for the calculation 

 of the total number. For instance (see form , pp. 302-3) 

 we might have on the first plate 43 examples of Melosira, 

 and as we know that the fraction of the total volume taken 

 for the purpose of calculation was one ten-thousandth part 

 of the whole, the total number of Melosira would be, 

 according to this single estimation, 430,000. The tenth 

 part of the square root of this is 66. When at least 66 

 specimens of Melosira have been counted, we can regard 

 the result as sufficient to supply us with an average, and no 

 more need be counted. In order to find out the degree 

 of correctness of the enumeration, Hensen adopted the 

 following method : — An organism is first of all counted 

 on several plates, and the average is taken. Then another 



