56 MICRO-ORGANISMS AND FERMENTATION. 



determine their multiplying capacity. The hsematimeter 

 consists, as shown in the diagram, of an object glass on which 

 a cover-glass of known thickness (0-2 mm.) is cemented, and 

 from the centre of which a disc has been cut out. A small 

 drop of the liquid containing the cells is brought into the 

 cavity thus formed, a second cover-glass is placed over the 

 opening, and thus rests on the cemented and perforated 

 cover-glass. The drop of liquid must not be so large that 

 the pressure of the cover-glass causes it to flow out from 

 the enclosed space, yet it must be high enough to be in contact 

 with the cover-glass. The thickness of the layer of liquid is 

 then known. In order to determine the other two dimensions, 

 and thus be able to work with a given volume of liquid, one 

 of the well-known forms of micrometer is introduced into the 

 eye-piece of the microscope. It may consist of a thin piece 



Fig. 13. Haematimeter a, object-glass; b, cemented cover-glass with circular opening ; 



c, cover-glass. 



of glass on which 16 small squares are engraved. The actual 

 value of each of these squares is known when a given system 

 of lenses is employed, and thus, when the square is projected 

 on the object, a small prism of known volume is defined. In 

 certain cases it may be more expedient to make use of an 

 appliance constructed by Zeiss, of Jena, from the instructions 

 of Thoma, which consists of a fine system of squares of known 

 size, engraved on the object-glass itself at the bottom of the 

 cavity. This also improves the microscopical definition of 

 the cells which are on the bottom of the chamber. 



When it is merely desired to determine the rapidity with 

 which the cells multiply, or to make repeated observations of 

 the number of cells in the same volume, it is quite superfluous 

 to determine its size ; it is simply necessary to work always 

 with the same volume. 



