32 PHARMACEUTICAL BOTANY 



3. Make two of the lines on the ocular micrometer coincide with 

 two on the stage micrometer. Note the number of included divi- 

 sions. 



4. Note the known value for each division of the stage micrometer 

 scale which may either be etched on the stage micrometer or indi- 

 cated on a label found pasted upon it. If the value indicated is o.oi 

 mm. (Jf oo mm.) then each division of the stage micrometer scale 

 has a value of 10 microns; if o.i mm. (Jfo mm.), 100 microns. 



5. Multiply'the number of included divisions of the stage microm- 

 eter scale by the value in microns given for each division and divide 

 the result by the number of included divisions of the ocular microm- 

 eter scale. The quotient represents the value of each division of 

 the ocular micrometer scale. 



6. Note the optical combination (number of ocular, objective and 

 tube length) used and keep a record of it with the calculated microm- 

 eter value. Repeat for each of the combinations. 



To measure an object by this method read off the number of 

 divisions it occupies of the ocular micrometer scale, and express the 

 result in microns by looking up the recorded value for the optical 

 combination used. 



