1892.] MICROSCOPICAL JOURNAL. 59 



pipette and transferred to a cell 50 by 20 millimetre area, and exactly 

 I millimetre in depth. Such a cell of course contains 1000 cubic milli- 

 metres, or I c.c. The top of the metal cell is ground perfectly smooth 

 and with a little practice one can float a thick cover-glass to place 

 without losing a drop. 



The next step is the enumeration. This is accomplished by trans- 

 ferring the cell to the stage of a microscope, the eye-piece of which is 

 fitted with a micrometer so ruled as to cover, with a given objective 

 and fixed tube length, a square millimetre on the stage. The microscope 

 itself is fitted with a mechanical stage with millimetre movement in 

 both directions; and for this purpose I have made certain simple 

 additions to the new mechanical stage of theBausch and Lomb Optical 

 Company, by means of which the desired result is obtained at slight 

 expense. The count is made by beginning at one corner of the cell 

 and going systematically over the area in accordance with such a formula 

 as will insure the count of squares selected from every part of the slide. 

 The number of squares actually counted will depend upon the degree 

 of accuracy which it is desired to attain. It is obviously impossible to 

 count the 1000 squares composing the entire area of the slide, and the 

 practical question arises as to just what multiple of 1000 shall be used 

 to secure a correct average. This can only be determined by trial and 

 comparison upon a number of samples. In any case, not less than 20 

 squares should be counted, and if time will possibly permit I should 

 prefer to always count at least 50. 



In order to illustrate the matter, I have prepared a table which 

 represents the area of the cell divided into 1000 squares. Brief inspec- 

 tion of this table will show the difficulty of obtaining true averages 

 when only 20 squares are counted, and exhibits clearly the value of 

 counting the larger number if one cares for true averages. (See Table, 

 page 60. ) 



I consider the precise millimetre movement of the mechanical stage 

 a matter of considerable importance, and indeed insist upon it as 

 an integral part of the method. Without it the tendency will be to 

 sometimes select squares for counting which are contiguous, while at 

 other times one will pass over squares containing few or no organisms 

 in a search for more prolific ones, making, in either case, an error in 

 the final result. By use of the mechanical stage with a definite formula 

 for passing over the slide," personal errors of this sort are eliminated, 

 leaving only those which are due to irregularity of distribution of the 

 organisms in the water, and by always stirring thoroughly before taking 

 the portion for examination with i c.c. pipette this error may also 

 be reduced to a small degree, provided as many as 50 squares are 

 counted as the basis of the final average. Additional uniformity of 

 distribution of organisms in the cell may also be obtained by stirring 

 gently in the cell itself with the pointed end of the pipette, before 

 floating the cover-glass to place, but the precaution should always be 

 taken in these stirring operations to proceed gently in order to guard 

 against breaking up unnecessarily the particles of amorphous organic 

 matter which are nearly always present in any sample of water in which 

 algous growths and decay are taking place. 



The definite estimation of the amorphous organic matter is a thing 

 of^me difficulty, and in my own use of this 'method I have formed a 



