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 (0.001 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 (0.1 mm.) or the entire surface 
of the glass may be etched with squares (0.1 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 (0.01 mm.), in some cases to tenths 
of a millimeter (0.1 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. 
