If  the  counting  slide  has  the  Thoma  ruling,  hence  but  i  sq.  mm.
for  use,  this  area  from  at  least  eight  diflferent  drops  should  be  counted.
The  Ewing,  Zappert,  or  Tiirk  rulings  are  to  be  preferred,  since  9
sq.  mm.  from  each  drop  can  be  counted.  This  should  be  repeated  with
three  diflferent  drops.  At  least  one  hundred  leucocytes  should  be
actually  counted,  and  more  if  possible.  If  the  acetic  acid  be  of  the
proper  strength  and  fresh,  and  the  pipette  clean,  all  objects  seen  may
be  counted.    The  number  of  cells  found  divided  by  the  number  of  units

27


418  CLINICAL  DIAGNOSIS

counted  and  multiplied  by  lo,  and  this  by  the  dilution,  will  give  the
number  of  leucocytes  in  i  cmm.  of  undiluted  blood.
Beware  of  nucleated  reds,  since  their  nuclei  are  similar  to  small
mononuclears.  The  hour  of  the  count  should  always  be  stated,  and
also  whether  a  short  time  before  the  count  the  patient  had  partaken
of  a  heavy  proteid  meal.
