objective  lens  varies  according  to  which  of  the three objectives is in focusing position. Thus, the intervening  distance  with  the  low-power  objective (10X)  is  the  greatest  (16  mm),  that  for  the  oil- immersion lens (100X) is the smallest (1.8 mm), and that for the high-power objective (45X) is in- termediate  (4  mm).  As  a  result,  the  focusing operation must be conducted with skill to avoid damage  to  the  objective  lens,  the  specimen,  or both.  It  is  good  practice  to  obtain  a  focus  with the low-power objective first, then change to the higher  objective  required.  Most  modern  micro- scopes   are   equipped   with   parfocal   objectives, which means that if one objective is in focus, the others  will  be  in  approximate  focus  when  the nosepiece is revolved. With the low-power objec- tive  in  focusing  position,  observe  the  following steps  in  focusing. 1. Seated behind the microscope, lower your head  to  one  side  of  the  microscope  until your eyes are approximately at the level of the stage. 2.  Using  the  coarse  adjustment  knob,  lower the  body  tube  until  the  face  of  the  objec- tive is within 1/4 inch of the object. Most microscopes  are  constructed  in  such  a  man- ner  that  the  low-power  (green)  objective cannot be lowered to make contact with the object  on  the  stage. 3. While looking through the ocular, use the coarse  adjustment  knob  to  elevate  the  body tube until the image becomes visible. Then use  the  fine  adjustment  knob  to  obtain  a clear and distinct image. Do not move the focusing  knob  while  changing  lenses. 4.  If  the  high-power  objective  (yellow)  is  to be  used  next,  bring  it  into  position  by revolving  the  nosepiece  (a  distinct  “click” indicates  it  is  in  proper  alignment  with  the body tube). Use the fine adjustment knob only  to  bring  the  object  into  exact  focus. Of course, light adjustment must be made; open the iris diaphragm of the condenser to  accommodate  more  light. 5.  The  oil-immersion  objective  (red)  is  used for  detailed  study  of  stained  blood  and bacterial smears. Remember that the dis- tance between objective lens and object is very   short, and   great   care   must   be employed.  After  focusing  with  the  high- power  objective  and  scanning  for  well defined  cells,  raise  the  objective,  place  a small  drop  of  immersion  oil,  free  of  bub- bles, on the slide, centering the drop in the circle  of  light  coming  through  the  con- denser.  Next,  revolve  the  nosepiece  to  bring the oil-immersion objective into place, and by means of the coarse adjustment knob, slowly lower the body tube until the lens just makes contact with the drop of oil on the slide. The instant of contact is indicated by a flash of light illuminating the oil. The final step in focusing is done with the fine adjustment knob. It is with this lens in par- ticular that lighting is important; the final focus,  clear  and  well-defined,  will  be  ob- tained only when proper light adjustment is made. CARE  OF  THE  MICROSCOPE The microscope is an expensive and delicate instrument  that  should  be  given  proper  care. Moving  or  transporting  the  microscope  should be done by grasping the arm of the scope in one hand and supporting the weight of the scope with the  other  hand.  Avoid  sudden  jolts  and  jars. Make sure the microscope is kept clean at all times; when not in use, enclose it in a dustproof cover  or  store  in  its  case.  Remove  dust  with  a camel hair brush. Lenses may be wiped carefully with lens tissue. When the oil immersion lens is not being used, remove the oil with lens tissue. Use oil solvents, such  as  xylol,  on  lenses  only  when  required  to remove dried oil and only in the minimal amount necessary. Never use alcohol or similar solvent to clean lenses. COMPLETE  BLOOD  COUNT The  complete  blood  count  consists  of: .  Total  red  blood  cell  count  (RBC) .   Hemoglobin   determination .   Hematocrit   reading .  Total  white  blood  cell  count  (WBC) .  Differential  white  blood  cell  count RED  BLOOD  CELL  (ERYTHROCYTE) COUNT The red blood cell count is made to determine the  number  of  red  cells  in  one  cubic  millimeter 6-5