MICROMETRY MICROMETRIC MICROSCOPES 



179 



The drums D 1 , D 2 , 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 M 2 ; 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 2 , 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 

 D 1 . Microscope B is focused by means of the pinion F, light 

 being thrown by the mirror through the preparation to be 

 examined. 



