Quality control methods for

Measurement of specimens 
 
Equipment 
Use a microscope with an ocular micrometer to measure the size of small objects. 
The scales should be calibrated using a stage micrometer, consisting of a glass 
slide of usual size, upon which a scale is engraved or photographed, usually 1 or 
2 mm long, in 0.1 and 0.01 mm divisions. The ocular micrometer consists of a 
small disc of glass, across the diameter of which a 100-line scale is engraved or 
photographed. The disc is placed into the eyepiece. 
Calibration of the ocular micrometer 
Place the ocular micrometer scale in the eyepiece of the microscope. Put the 
stage micrometer on the microscope stage and focus on the scale divisions. Both 
scales should be sharply defined in the field of view. Turn the eyepiece to place 
the scales in a parallel position and, if necessary, move the stage micrometer 
until the starting lines of both scales coincide (Fig. 1). Now find another point, as 
far along the scale as possible, where two other division lines are exactly 
superimposed. Count the number of divisions on the ocular micrometer and the 
corresponding length on the stage micrometer scale, in order to determine the 
length that is equivalent to one division on the ocular micrometer scale; for 
example, if 100 divisions on the ocular micrometer scale are equal to 30 divisions 
on the stage micrometer scale, since the divisions on the stage micrometer scale 
are 0.01 mm apart, 100 ocular micrometer divisions are equivalent to 0.30 mm, 
therefore each division on the ocular micro-meter scale represents 3.0 µm. Since 
the calibrations apply only for a particular lens combination, it is advisable to 
determine and record the ocular micrometer values for the most frequently used 
combinations. 

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Quality control methods for medicinal plant materials