MICROSCOPICAL AND PHYSIOLOGICAL EXAMINATION. 35 



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 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, e.g., a thin 

 piece of glass on which 16 small squares are engraved, is intro- 

 duced 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, 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. 



It is always necessary that the sample taken should be a 

 fair average one. In most cases it must be diluted and 

 thoroughly agitated for a long time, in order to obtain an equal 

 distribution of the cells ; the specific gravity of the liquid must 

 also be such that it will allow the cells to remain suspended in 

 it for a short time. A small drop is then withdrawn in a 

 capillary tube, transferred to the counting apparatus, and 

 covered with the cover-glass. The apparatus is allowed to 

 remain at rest for some time, in order that the cells may settle 

 to the bottom of the enclosed space, and on this account the 

 specific gravity of the liquid must not be greater than will 

 allow this to take place in a convenient time. Both these 



