156 THE AMERICAN MONTHLY [iMny 



needs but a slide with ii tlat ctdl cemeiitHd on it, and a 

 plate cover glass in order to do tlie work. Ferliaps the 

 most convenient thing is the slide that goes with the 

 Z^iss-Thoma blood counter. This has a circular cell 

 cemented onto the slip, with a central cross lined disc, 

 which forms an elevated platfoiin in the centre of the 

 cell, leaving a groove to catch any excecs of liquid, in 

 order that it may not flow between the top of the cell and 

 cover glass. 



In making the examination, the rules accompanying 

 this instrument must be strictly regarded, in order to in- 

 sure direct contact with the cover and top ot cell. That 

 is, when a minute drop has been placed on the platform 

 and covered with the accompanying i)late glass cover, 

 the newtonian rings must appear, otherwise a hit of dust 

 or something has prevented perfect contact between cell 

 and cover. The depth of the cell in this instrument is 

 convenient for calculations, as it is exactly 10 microns. 



Procedure. Dust carefully the cell and cover glass 

 with a soft lens brush. After putting the slip on the 

 stage of the microscope (under a 1-5 or D objective for 

 convenience in accurate focussing) the cover is put in place 

 with a jiair of forceps, pressed down centrally with the ball 

 of the finger. The finger print made is of the greatest 

 use. If the Newtonian rings are apparent, allis well; if 

 not, try again. Turning the z-ero mark on the m. m. fine 

 adjustment sci'ew to the })ointer, focus to the to}) of cover 

 glass with coarse adjustment. A little patience allows 

 one to do this, and it is much more convenient. This 

 done, focus with the fine adjustment, noting the distance 

 on the m. m. scale, until the top of the cross lines of the 

 counter are in perfect focus. This distance represents 

 the depth of cell, })lus cover glass eijual m. Removing the 

 cover and pressing between the fingers, focussing on top 

 and on bottom gives apparent (svliich is all re(|uired,) 

 thickness of cover equal u. 



