LABORATOKY AND TEACHING METHODS 



583 



5. Nole the optic combinations used and keep a record of them with the calcu- 

 lated micrometer value. Repeat for each of the other combinations. To meas- 

 ure an object by this method, read off the number of divisions of the eyepiece 

 micrometer it occupies and express the result in microns by looking up the standard 

 value for the optic combination used. 



Example. — Determine how many of the stage micrometer divisions correspond 

 with the eyepiece micrometer divisions. Divide the first by the last, the quotient 

 will be the true value of the ocular micrometer divisions in units of the objective 

 micrometer. If 20 divisions of the ocular micrometer cover 87 divisions of the 

 stage micrometer then ^%o = 43-5 = 0.0435 mm. 



Method uith Filar Micrometer (Fig. 207). — This consists of an ocular having a 

 fixed wire stretching horizontally across the field with a vertical reference wire 



Fig. 207. — Screw micrometer eyepiece (Filar micrometer). 



adjusted at right angles to the first and a fine wire, parallel to the reference wire, 

 which can be moved across the field by the action of the micrometer screw. The 

 trap head is di^^ded into 100 parts, which pass successively a fixed index as the head 

 is turned. A fixed comb with the intervals between its teeth corresponding to one 

 complete revolution of the screw head is found in the field. As in the previous 

 method, the value of each division of the comb scale must be found for each optic 

 combination. 



1. Place the filar micrometer and the stage micrometer in their respective 

 positions. 



2. Rotate the screw of the filar micrometer until the movable wire coincides with 

 the fi.xed one, and the index marks zero on the screw head. 



