MICROMETRY — MICROMETRIC MICROSCOPES 
175 
Micrometry. — The measurements of minute objects or the 
determination of the magnitudes of microcroscopic dimensions 
are made with ordinary compound microscopes provided with 
measuring devices or by means of microscopes of special con¬ 
struction known as Micrometric Microscopes or Comparators. 
Micrometric Methods. — The methods which are generally 
applicable to the measurement of minute objects may be cori- 
veniently grouped as follows: 
1. Comparing the object directly with a standard scale laid 
in juxtaposition on the stage of the microscope within the field 
of vision: or comparison with a scale attached to the stage or 
adjacent to the stage. 
2. Measuring the object by means of a drawing camera 
and stage micrometer. 
3. Measuring the object by means of an ocular containing 
a scale of known value. 
4. Measuring the object by projecting into the field, by means 
of the substage condenser (or other suitable lens) the image of 
a scale of known value. 
5. Measurements obtained with the graduated head of the 
fine adjustment. 
At the present time substantially all measurements of micro¬ 
scopic objects are recorded in microns and universally desig¬ 
nated by the Greek letter /x. A micron is one-thousandth of a 
millimeter. In the case of submicroscopic objects, as, for exam¬ 
ple, the exceedingly minute particles demonstrated by the ultra¬ 
microscope, a still smaller unit becomes necessary in order to 
avoid the use of cumbersome figures. To meet this need the 
term submiCron or ultramicron has been proposed for a value 
equal to one-thousandth of a micron, the designation to be mm- 
All micrometric measurements with the compound microscope 
necessarily partake of the nature of close approximations; the 
more skillful and experienced the investigator the more nearly 
will the values obtained approach the true dimensions of the 
object. 
According to Rogers ^ it is impossible to obtain true values 
1 Rogers, W. A., Proc. Am. Soc. Micros., 1883, 198. 
