THE BACTERIOLOGICAL MICROSCOPE. 



81 



on a piece of paper, and a drawing made, and the object to be 

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

 scale. 



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

 are stretched across the field 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 to that of the 

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

 divided into one hundred parts, and a screw with one hundred threads 

 to the inch is used. The bacterium to be measured is brought into a 



FIG. 25. MICROMETER EYE-PIECE BY ZEISS. 



position in which one edge appears to be in contact with the fixed 

 wire, and the micrometer screw is turned until the travelling wire 

 appears to be in contact with the 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 by 

 means of a micrometer screw (Fig. 25). Each division on the edge 

 of the drum corresponds to '01 mm. Complete revolutions of the 

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

 Another method of measuring bacteria will be referred to in the 



