CHAPTER VII. 

 MICROMETRY MICROMETRIC MICROSCOPES. 



The methods commonly employed for the measurement of 

 minute objects by means of the microscope may be conveniently 

 grouped under six different heads. 



1. Comparing the object with a standard scale laid in juxta- 

 position on the stage of the microscope within the field of vision. 



2. Measuring the object by means of a camera lucida and 

 stage micrometer. 



3. Measuring the size of the real image of the object in the 

 microscope and dividing by the magnification of the instrument. 



4. Measuring the object by means of an ocular micrometer, 

 the value of whose scale divisions are known. 



5. Measuring the object by projecting into the field, by 

 means of a substage condenser, a scale of known value. 



6. Measurements obtained by 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 p. 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 ju/*. 



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. 



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