BLOOD 267 



minute in order to mix the blood thoroughly 

 with the diluent. The blood will now be diluted 

 200 times its volume. 



Eemove the rubber tube from the pipette. 

 Blow out the unmixed solution in the capillary 

 tube, between the point and the bulb, and several 

 drops of the mixture in the bulb. Wipe off the 

 end of the pipette. Touch it to the ruled disc. 

 Let a very small drop flow out. Place the cover 

 glass on the drop. The flattened drop should 

 almost cover the glass. If it spread into the 

 moat, clean the disc and use a second, smaller 

 drop. If Newton's color-rings cannot be seen 

 between the cover-glass and the disc by placing 

 the eyes near the level of the cover-glass, another 

 preparation must be made, with cleaner disc and 

 cover-glass. 



Use Leitz No. 5 or Zeiss D objective. Bring 

 the drop into focus and then, using the microm- 

 eter screw, find the ruled field. 



On the central portion of the disc 1 square 

 millimetre has been ruled into 400 squares, each 

 square having therefore an area of ^-j-g- square 

 millimetre. Each 16 small squares are sur- 

 rounded by double lines, thus forming a " large 

 square." In the Zappert-Ewing slide, the cen- 

 tral square of 1 mm. is surrounded by eight other 

 squares of 1 mm. each, and the central ruling is 



