MICROSCOPICAL AND PHYSIOLOGICAL EXAMINATION. 33 



The counting is performed by means of an apparatus 

 constructed by Hayem and Nachet (Fig. 7), which was first 

 employed for counting the corpuscles of blood (hence termed 

 hcematimeter). The late Prof. Panum, of Copenhagen, was 

 the first to employ this apparatus for counting micro- 

 organisms, in order to determine their multiplying capacity. 

 The hsematimeter consists, as shown in Fig. 7, of an object- 

 glass on which a cover-glass of knoivn thickness (0*2 mm., 

 for instance) 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 cover-glass 

 is placed over the opening, and thus rests on the cemented 

 and perforated cover-glass. The drop of liquid must not be 



FIG. 7. 



Hsematimeter : a, object-glass ; b, cemented cover-glass with circular opening ; 



c, cover-^ 



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 



o 



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 generally known micrometers, 

 e.g., a thin piece of glass on which 16 small squares are 

 engraved, is introduced into the eye-piece of the microscope. 

 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, and which consists of a fine 



