170 A MANUAL OF BACTERIOLOGY 



in the eye-piece after unscrewing the top lens. The value 

 of the divisions in this eye-piece 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 

 eye-piece scale. If the objective or the eye-piece be 

 changed or the tube -length altered the value of the 

 divisions of the eye-piece scale will be altered, and must 

 again be determined by means of the stage micrometer. 

 The eye-piece micrometer is a more elaborate device of the 

 same nature as the eye-piece scale. It consists of an eye- 

 piece in which two fine filaments can be adjusted by 

 means of screws so as just to correspond with the limits 

 of the object. This having been done, and without 

 altering any of the adjustments, the object is removed 

 and replaced with the stage micrometer, and the distance 

 between the two filaments is then determined. Various 

 accurately divided scales both for eye-piece and for stage, 

 reproduced photographically and burnt into the glass 

 (" graticules ") are made by Messrs. Rheinberg. 



The unit for microscopical measurement is the micron 

 (sometimes erroneously termed a micro-millimetre), which 

 measures one-thousandth of a millimetre, or approximately 

 "2"7>ArFO" f an mcn > and is designated by the sign /A. 



If the 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 in diameter. 



