186 



ELEMENTARY CHEMICAL MICROSCOPY 



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 magni- 

 fication may fail to yield real images of sufficient size to even 

 fill a single division of the ocular scale. To meet conditions 

 such as these filar micrometers are employed. In instruments 

 of this kind, a set of cross-hairs are made to traverse a fixed 

 scale by means of a screw provided with micrometer thread, 

 the amount of the movement of the cross-hairs being indicated 

 by the revolution of a drum attached to the screw head. Typical 

 instruments of this class of micrometer oculars are shown in 

 Fig. 1 20 and Fig. 121. The scales and measuring devices of 

 instruments of this class differ in different instruments. 



Before filar micrometers may be used for micrometry the value 

 of one division of the ocular scale must be ascertained by means 

 of a stage micrometer with the draw-tube of the microscope in a 

 recorded position. 



When using micrometers in which the diameter of the image 

 of the object is measured by the movement of a micrometer 

 screw, a number of observations should be made, always moving 

 the cross-hairs in the same direction to eliminate " back-lash." 



To measure the length of an object by means of a microm- 

 eter eyepiece of the type shown in Fig. 120 first set the drum 



FIG. 1 20. Spencer Lens Co. Filar Micrometer. 



of the micrometer screw at o, move the preparation until an 

 edge of the image of the object is in contact with o on the scale. 

 Count the number of whole divisions of the scale seen in the 



