154 



ELEMENTARY CHEMICAL MICROSCOPY 



corners. A scale of this type will stand out sharply, no matter 



how bright the object may be. 



Filar Micrometers. In micrometry with oculars having fixed 



scales there is always the probability of considerable error, as we 



have seen, since the magnitude of 

 the real image as measured by the 

 ocular scale usually requires a guess 

 as to just how much of the scale is 

 included. Very minute objects even 

 with high magnification may fail to 

 yield real images of sufficient size to 

 even fill a single division of the oc- 

 ular scale. To meet conditions such 

 as these filar micrometers are em- 

 ployed. In instruments of this kind, 

 a set of cross-hairs are made to 



FIG. 97. Zeiss Contrast Microm- 

 eter Ocular for Measuring Bright 

 Bodies. 



traverse a fixed scale by means of a screw provided with microm- 

 eter thread, the amount of the movement of the cross-hairs 

 being indicated by the revolution of a drum attached to the screw 

 head. A typical instrument of this class of micrometer oculars 



FIG. 98. Bausch & Lomb Filar Micrometer Eyepiece. 



is shown in Fig. 98. The scales and measuring devices of instru- 

 ments of this class differ in different instruments. Figs. 99 and 

 100 illustrate two of these forms. The Zeiss filar micrometer has 

 an immovable scale with the zero at a fixed point as shown, Fig. 

 99; across this scale there moves simultaneously the set of cross- 



