MICROMETRY — MICROMETRIC MICROSCOPES 
179 
The drums D\ D^, are set at zero; the point or line on the 
object from which measurements are to start is brought exactly 
under the cross-hairs of B by means of the exact position 
with respect to the scale is determined by the reading microscope 
A with great accuracy, using the ocular micrometer. The entire 
stage is next displaced, by hand or by the screw M^, to the right 
Fig. 114. Comparator. Wm. Gaertner & Co. 
or left, as the case may be, until the second point is reached and 
the scale again read. 
In order to vary the magnification of the observing microscope 
B, the objective is mounted so as to slide up and down in the 
body tube. A double-ended pointer P attached to the objective 
mounting moves over two scales, one of which indicates the 
magnification, the other the ocular micrometer value of the drum 
Db Microscope B is focused by means of the pinion F, light 
being thrown by the mirror 0 through the preparation to be 
examined. 
