MORPHOLOGY OF LEUCOCYTES AND RED CORPUSCLES 255 



left hand the drop of blood on the patient's skin, so as to 

 remove a very small droplet. This is the most difficult step; 

 you must not get too much or too little blood, otherwise the 

 films will be useless. It is advisable to avoid letting the 

 cover-glass touch the patient's skin. 



Now put the right-hand cover-glass over the left-hand one, 

 the centres coinciding; lower the upper (right-hand) one until 

 the droplet of blood just touches it, and then let go (Fig. 52). 



You will see the droplet of blood spread its-elf out by capil- 

 lary attraction between the two cover-glasses. 



At this stage you will see whether you have taken the right 

 amount of blood or no. If you have, the drop will spread 

 out, still retaining its circular shape, until it approaches the 

 octagon formed by the intersecting edges of the two cover- 



FIG. 51. 



glasses (Fig. 52) ; if you have taken too little it will not reach 

 so far, and if you have taken too much it will extend farther, 

 and the upper cover-glass will float loosely on the lower. 



It is necessary to lay great emphasis on the fact that the 

 cover-glasses must not be squeezed together, but must simply 

 come together by capillary attraction. 



When the drop has ceased to spread take hold of the upper 

 cover-glass with the finger and thumb of your right hand and 

 slide the two apart, keeping them in the same plane; this is 

 readily done, since the cover-glasses are free to turn, being 

 only held loosely at the points. If you do not do this, either 

 the cover-glasses will break, or else the upper cover-glass be 

 lifted from the lower one, and the film will resemble the marks 

 left on a knife which has been pressed on butter and lifted off; 

 such films are useless. 



