ZOOLOGY AND BOTANY. MICROSCOPY, ETC. 



503 



with a circular aperture upon an object slide, and cementini:!' a smaller 

 plate of less thickness into this aperture. The surface of hoth plates 

 being parallel and ground accurately plane, the depth of the chamber, or 

 distance between the two surfaces, is regulated by selecting plates of 

 desired thickness. By placing a plane cover-glass upon the outer plate, 

 a piano-parallel cavity, the dimensions of which can be accurately deter- 

 mined, is formed between the cover-glass and the inside plate. This 

 inside plate is ruled with cross-lines by means of which the observer is 

 enabled to successively examine fluids on separate fields, and to count 

 the corpuscles in each. These cross-lines are designed of different forms 



Fig. 85. 



to facilitate the counting of both red and white corpuscles ; the latter 

 being considerably fewer than the former in a given volume of blood, 

 the areas to be counted should vary in size. The counting chamber 

 advocated by Fuchs and Rosenthal for the cytological investigation of 

 cerebrospinal fluids has a depth of ' 2 mm. The simple plan of cross- 

 lines is shown in the figure (fig. S.i). The mixing pipettes consist 

 of an accurately caHbrated capillary tube dilated above to form a cavity, 

 and provided with a rubber suction tube. The capillary tube is 

 graduated, the uppermost mark being 1, and the 101 mark being just 

 above the cavity ; so that if the fluid to be examined is drawn in up to 



