138 BACTEEIOLOGY. 



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 t^ mm. , it follows that 1 division 

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

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

 micron, and is designated by the letter /j.. The micron is 

 approximately FE^irth of an inch. Therefore, each division 

 -on the ocular micrometer, used in connection with this same 

 eye-piece and objective, corresponds to 15.5 /j.. 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 fi. 



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 = j^i = .0036 mm, =3.6 /j-. Again, 

 in case of the ^ 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 ^ = 

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

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



