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. 120 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. 120. Spencer Lens Co. Filar Micrometer. 
of the micrometer screw at o, move the preparation until an 
edge of the irnage of the object is in contact with o on the scale. 
Count the number of whole divisions of the scale seen in the 
