THE BACTERIOLOGICAL MICROSCOrE. 



n 



ou a piece of paper, and a drawing made, and the object to l)e 

 measured can tlien be projected on the paper and compared with the 

 scale. 



In the Eamsden micrometer eye-piece (Fig. 24) two fine wires 

 are stretched across the tiehl of an eye-piece, one of which can be 

 moved by a micrometer screw. In the field there is also a scale 

 with teeth, and the interval between them corresponds tci that of the 

 threads of the screw. The circumference of tlie brass head is usually 

 chvided into one hundred jaarts, and a screw with, one hundred tlu-eads 

 to the inch is used. Tlie bacterium to be measureil is brought into a 



Fig 



-MiCKOMETER EVE-PIECE BY ZeISS. 



position in which one edge appeai-s to be in contact with the fixed 

 wire, and the micrometer sci-ew is turned until the travelling ■sriie 

 appeai-s to be in contact with tlie other edge. The scale in the 

 field and the scale on the milled head together give the number of 

 complete turns of the screw and the value of a fraction of a turn in 

 separating the wires. 



In the micrometer eye-piece constructed by Zeiss, the eye-piece 

 with a glass plate with crossed lines is carried across the field l)y 

 means of a micrometer screw (Fig. 25). Each ch\'ision on the edge 

 of the drum coiTe.sponds to "01 mm. Complete revolutions of the 

 drum are counted by means of a figured scale in the visual field. 

 Another method of measuring bacteiia wdll be referred to in the 



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