are  also  commonly  used.  The  microscope  most often  used  in  the  laboratory  is  a  compound microscope  that  consists  of  the  vorious  pieces identified  and  discussed  briefly  below: 1. 2. 3. Framework: Base—structure  on  which  the  microscope rests. Arm—structure   that   supports   the magnification and adjustment system; it is the  handle  by  which  the  microscope  is carried. Stage—platform on which a preparation is placed  for  examination.  In  the  center  of  the stage   is   the   aperture   or   hole   to   allow passage  of  light  from  the  condenser. Mechanical  stage—means  by  which  the preparation  may  be  moved  about  on  the stage. Illumination System: Mirror—usually  double,  a  flat  surface  on one  side,  and  a  concave  surface  on  the other side. The concave surface is used in the   absence   of   a   condenser. Many microscopes  have  a  built-in  light  source  in- stead  of  a  lamp  and  mirror. Internal light source—built  into  the  base of  the  microscope,  and  provides  a  more precise   steady   source   of   light   into   the microscope. Condenser—composed  of  a  compact  lens system  located  between  the  mirror  and stage.  The  condenser  (usually  an  Abbe  con- denser) concentrates (condenses) the light through the aperture in the stage to the ob- jective lens. Iris  diaphragm—controls  the  amount  of light  reaching  the  condenser.  The  size  of the iris diaphragm opening should approx- imate that of the face of the objective lens. Thus, as a general rule, the diaphragm is completely  closed  when  liquid  preparations are observed with the low-power objective, and wide open when stained preparations are  observed  with  the  oil-immersion  lens using natural light. Magnification  System: Revolving   nosepiece—contains   openings into  which  objective  lenses  may  be  fitted and that may be revolved to bring an ob- jective  into  the  desired  position. Objective  lenses— usually  a  set  of  three consisting  of  a  low-power  lens  (approximate 6-4 focus 16 mm, magnification 10X), a high- power  lens  (approximate  focus  4  mm, magnification 45X), and an oil-immersion lens (approximate focus 1.8 mm, magnifi- cation   100X).   Numerical   aperture   (NA) refers  to  the  angle  of  the  maximum  cone of light that may enter the objective. The greater  the  numerical  aperture,  the  greater the resolution, or ability of the microscope to separate small details clearly. The  body  tube—through  which  light  passes from  the  objective  to  the  ocular  lens. The  ocular  lenses  (eyepieces)—usually  a 10X is provided: the number indicates the magnification  (in  diameters)  produces  by the ocular of the image formed by the ob- jectives.  Magnification  is  determined  by  the ratio between the size of the virtual image and  the  real  size  of  the  object.  It  is expressed  in  diameter  multiples,  for  exam- ple  100X.  By  multiplying  the  magnification engraved  on  the  objective  by  that  engraved on the eyepiece, one can determine the total magnification. The  total  magnification resulting  from  the  systems  of  lenses  is determined  by  the  combination  of  objec- tives and oculars: 4.   Adjustment   System   (composed   of   two parts, both of which raise or lower the body tube together with the lens system): Coarse adjustment —the larger and inner- most  knob;  by  rotating  the  control  knob, the image appears and is in approximate focus. Fine adjustment —the smaller and outer- most knob; by rotating this control knob, it renders the image clear and well-defined. FOCUSING  THE  MICROSCOPE The process of focusing consists of adjusting the  relations  between  the  optical  system  of  the microscope and the object to be examined so that a  clear  image  of  the  object  is  obtained.  The distance between the upper surface of a glass slide on  the  microscope  stage  and  the  faces  of  the