LABORATORY AND TEACHING METHODS 



619 



Sample i 



Cells in each large square 



Calailatioti of Counts. — As these four averages are nearly the same, it is not 



necessary to count more drops. The mean of the four averages is ' = 20.275 



4 

 cells per unit of volume. But since the wort was diluted with H2SO4 (4 parts of 

 the mixture contains 3 parts of wort with cells) the actual number of cells in the 



, . ,. . 20.275X4 „ 



volume m question is = 27 cells. 



Detailed Description of Thoma's Hamatimeter (Figs. 218 and 218A). — ^Thoma's 

 haematimeter (Zeiss form) is used also for counting microorganisms. .4 is a glass 

 slide on which a cover-glass (a) is fastened which has a circular hole in the middle 

 and is 0.2 mm. thick. A circular cover-glass (c), o.i mm. thick is fitted centrally 

 in this hole and is also fastened to the glass slide; thus an annular space {d) is formed. 

 In the middle of (c) two sets of parallel lines are etched which cut each other at 

 right angles. There are thus formed a large square with a side of i mm., and small 

 square with a side of 0.005 mm. The drop of liquid to be examined is placed on 

 this square and enclosed by the cover-glass (6), the depth of the liquid layer (e) 

 thus formed amounting to o.oi mm. B gives a vertical section of the chamber. 



If the actual number of cells in a certain volume is to be calculated, the size of 

 the space unit must be determined. It is then necessary to know the height of the 

 column of Hquid, i.e., the thickness of the perforated cover-glass. The haematimeter 

 designed by Hayem and Nachet has one with a thickness of 0.2 mm., but that in the 

 Zeiss hasmatimeter is usually o.i mm. The value of the square in the "net" for 

 the magnification used must further be known, or squared cover-glasses are used of 

 which the size of the squares is known. In Thoma's chamber the column of liquid 

 is 0.1 mm. high and the large square etched on the bottom of the chamber contains 

 I sq. mm. The volume of the liquid prism, of which the base is the large square, is 

 thus 0.1 cu. mm. 



