FUNDAMENTAL CONSIDERATIONS 31 



in the tissues has disappeared. Stop the action by pouring the 

 whole ' of the contents into a vessel containing water and wash 

 well with water. The cells can now be readily separated with 

 dissection needles and mounted in water for examination. Do not 

 mount in glycerine, for it makes the already bleached elements too 

 transparent. 



MICROMETRY 



The unit of length used in microscopic measurement is the micron 

 (jit) which is one-thousandth part of a millimeter (o.ooi mm.) or one 

 twenty-five thousandth part of an inch. 



In measuring microscopic objects it is necessary to make use of 

 a micrometer of some kind. That pretty generally used is the 

 ocular micrometer. It is a circle of glass suitable for insertion 

 within the ocular with a scale etched on its surface. The scale is 

 divided to tenths of a millimeter (o.i mm.) or the entire surface 

 of the glass may be etched with squares (o.i mm.), the net 

 micrometer. 



STANDARDIZATION OF OCULAR MICROMETER 



The value of each division of the ocular micrometer scale must be 

 ascertained for each optical combination (ocular, objective, and 

 tube length) by the aid of a stage micrometer. 



The stage micrometer is a slide with a scale engraved on it divided 

 to hundredths of a millimeter (o.oi mm.), in some cases to tenths 

 of a millimeter (o.i mm.), every tenth line being made longer than 

 intervening ones, to facilitate counting. 



METHOD : 



1. Insert the ocular micrometer within the tube of the ocular by 

 placing it on the diaphragm of the ocular, and adjust the stage 

 micrometer by placing it on the stage of the microscope. 



2. Focus the scale of the stage micrometer accurately so that the 

 lines of the two micrometers will appear in the same plane. Make 

 the lines on the two micrometers parallel each other. This can 

 often be done by turning the ocular to the right or left while looking 

 into the microscope. 



