MEASUREMENT OF MICRO-ORGANISMS 143 



scale already drawn. A simpler and less expensive arrange- 

 ment is to make use of a disc of glass ruled with equi- 

 distant fine lines, which can be placed in the eyepiece by 

 unscrewing the top lens and dropping it on the diaphragm 

 below. The value of the divisions in the eyepiece scale 

 is first ascertained by means of the stage micrometer. 

 The stage micrometer is then removed and the object to 

 be measured put in its place, and its dimensions are 

 determined by means of the eyepiece scale. With the 

 eyepiece micrometer, the value of the divisions is first 

 ascertained by means of the stage micrometer, which is 

 then replaced by the object. If the objective or the eye- 

 piece be changed the value of the divisions of the eyepiece 

 scale in both cases will be altered, and must again be 

 determined by means of the stage micrometer. The unit 

 for microscopical measurement is the micron (sometimes 

 erroneously termed a micro-millimetre), which measures 

 one thousandth of a millimetre, or approximately 0^375- 

 of an inch, and is designated by the sign /m. 



If a micrometer is not available, rough measurements 

 may be carried out by comparison with a red blood- 

 corpuscle. The majority of the red corpuscles of normal 

 human blood measure 7-5 JUL in diameter. 



