222 THE AMERICAN MONTHLY t 0( 'toW, 



tant ; the contents of the capillary tube up to the mark i do riot enter 

 the mixture. In this case the proportion of the blood to the dilu- 

 tant will be i : ioo ; if the mixer be only filled to the mark 2 with blood 

 then the proportion will be i : 200. After use the mixer is washed out 

 with caustic potash, then well with water, and finally dried by draw- 

 ing a current of air through it. 



For counting the cells various instruments have been devised. Two 

 of the latest forms, those of Thoma and Malassez, give very exact re- 

 sults. Malassez's counter, Fig. 3, consists of a thick nickel slide, in 

 the centre of which is a circular groove enclosing a glass cylinder 1 

 cm. in diameter. On the outside of the groove, equidistant from each 

 other, are three pointed screws, which project exactly -^ of a millimetre 

 above the surface of the slide. In the centre of the slide limited by .the 

 groove are drawn the squares in which the blood cells are counted 

 (Fig. 4). These have a side of 4^ of a millimetre, and are arranged 

 in groups of twenty, each group having a length of J of a milli- 

 metre and a width of \ of a millimetre, and an area, therefore, of -jl- X £ ' 

 = 4q of a square millimetre. Each group is separated from the adjoin- 

 ing groups by double lines. The cover-glass, which is ground per- 

 fectly flat, is attached, by moistening the edges slightly with saliva, to 

 a frame fixed to the sides of the slide (Fig. 3). The frame is then low- 

 ered until it comes in contact with the screw points, thus spreading out 

 the drop of blood, previously placed on the sui'face of the glass cylin- 

 der, in a perfectly horizontal layer | of a millimetre in thickness. To 

 prevent evaporation a drop of water is allowed to run under the cover- 

 glass and fill the vacant space between its edge and the groove in the 

 surface of the slide. The slide is now placed on the microscope, and 

 with a power of 200 the red cells lying within a group of twenty squares 

 are counted. As these squares have an area of -fa of a square millime- 

 tre, and the thickness of the layer of fluid is \ of a millimetre, there- 

 fore the quantity covering the group of twenty squares will equal 4-^ X 

 -^ = t 1q- of a cubic millimetre. The number of cells counted, there- 

 fore, has to be multiplied by 100, and then again by the number repre- 

 senting the dilution, and the product will be the number of cells in a 

 cubic millimetre of undiluted blood. For example, the mixture used 

 has a dilution of r to 200, and the number of cells found in T ^ of a cu- 

 bic millimetre equals 250; then 250 X 100 X 200= ^,000,000. Thus, 

 to the number of cells counted add 0000, if the dilution be 1 to 100 ; if 

 the dilution be greater, multiply the number of cells counted by the 

 figure representing the dilution before adding the 0000. 



For counting the white cells the number found in ten of the rectan- 

 gles of twenty squares must be taken. If in a dilution of 1 to 100 the 

 number of cells counted be, for example, 30 ; then as the volume 

 of the mixture equals ^VX \ X 10 = -^ of a cubic millimetre, there- 

 fore the number of white cells counted must be multiplied by 10 and 

 then by 100, and the product will be the number of cells in a cubic 

 millimetre of undiluted blood. 



\To be continued.^ 



Dr. P. I. Leonard lectures once a week upon pathology and micros- 

 copy in the Ensworth Medical College, St. Joseph, Mo. He pays es- 

 pecial attention to the technique of microscopical work upon normal 

 and morbid tissues. 



