138 BACTERIOLOGY. 



aud an ocular micrometer. The former is a glass slide on 

 which a scale of 1 mm. is subdivided into 100 parts. The 

 ocular micrometer is placed inside the eye-piece on the 

 diaphragm, by unscrewing" the eye-lens which is then re- 

 placed. It lies, therefore, in the same plane as the image 

 made by the objective. If this is not the case the dia- 

 phragm should be raised or lowered till the plane of the 

 ocular micrometer coincides with that of the image. It is 

 necessary, first, to ascertain for each objective the value of 

 a division of the ocular micrometer. 



The stage micrometer is focussed, first, with the No. 3 

 objective. It will probably be found that 10 divisions on the 

 scale in the eye-piece cover 15.5 divisions of the stage micro- 

 meter. 1 division of the ocular micrometer corresponds, 

 therefore, to 1.55 divisions on the stage micrometer. Since, 

 each of the latter represents OT mm., it follows that 1 division 

 on the scale in the eye-piece corresponds to \^ = 0.0155. The 

 TiJWth part of a mm., as explained on p. 24, is known as a 

 micron, and is designated by the letter p.. The micron is 

 approximately s^uth of an inch. Therefore, each division 

 on the ocular micrometer, used in connection with this same 

 eye-piece and objective, corresponds to 15.5 /*. If, now, an 

 object is placed under this No. 3 objective in place of the 

 stage micrometer, and it corresponds to 10 divisions on the 

 scale of the ocular micrometer, it is evident that the object 

 measures 155 //.. 



The " micrometer value " of the No. 7 objective is ascer- 

 tained in the same manner. Thus, if 50 divisions on the 

 ocular scale correspond to 18 divisions on the stage micro- 

 meter, 1 division of the former will correspond to 0.36 

 divisions of the latter = .0036 mm. =3.6 //.. Again, 

 in case of the iV inch objective, 50 divisions on the eye-piece 

 micrometer correspond to 8.6 divisions of the stage micro- 

 meter. Hence, 1 division of the former corresponds to y = 

 0. 172 of one division of the latter. Since each division of 

 the latter represents ita of a mm., one division of the eye- 



