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 con- 

 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 /*. 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 /*/*. 



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 l it is impossible to obtain true values 



1 Rogers, W. A., Proc. Am. Soc. Micros., 1883, 198. 



