LABORATORY AND TEACHING METHODS 



583 



$. Note 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 microus by looking up the standard 

 value for the optic combination used. 



Example.— Detetraine 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 ^Jio = 43-5 = 0.043S mm. 



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

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





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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 divided 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 fixed one, and the index marks zero on the screw head. 



