36 BACTERIOLOGICAL APPARATUS 
‘With an ordinary filar micrometer it would have been necessary 
to move the cross hairs over the whole length of the object, which in the 
above instance would have required nine full revolutions besides the 
fractional turning of the drum, while with our screw micrometer it 
was only necessary to move the scale over a part of one space. 
“For measurements of less accuracy, this micrometer can also be 
used without turning the screw, simply using the subdivisions of the 
scale as in the ordinary micrometer.” 
Zeiss Micrometer Ocular. This consists of a Ramsden ocular 
of about 20 mm. focal length. The glass plate, with a crossed as well 
Fig. 29 —Zeiss Micrometer Ocular. 
as a double line, is moved over the field by turning the micrometer 
screw. Each unit on the revolving drum corresponds to a move- 
ment of 0.01 mm. of the point marked. The absolute values of the 
units on the drum must be determined from a stage micrometer. 
Calibration of Micrometer Eyepiece. For this pull the draw tube 
to the proper length and focus the lines of an object micrometer sharply. 
This may be done with an ordinary ocular. Replace this with the 
micrometer eyepiece, and focus again so that the lines on both microm- 
eters are sharp. 
The object micrometer is made from a cover glass bearing a scale 
which is divided into a certain number of parts. These data are usually 
