134 



MA GNIFICA TION AND MICR OME TR Y [ CH. IV 



curate stage micrometer must be used. Carefully focus the y^ 

 mm. spaces. The lines of the ocular micrometer should also be 

 sharp. If they are not focus them by moving the top of the ocular 

 up or down ( 188). Make the vertical lines of the filar micrometer 

 parallel with the lines of the stage micrometer. Take the precau- 

 tions regarding the width of the stage micrometer lines given in 

 197 (see also Fig. 123). Note the position of the graduated 

 wheel and of the teeth of the recording comb, and then rotate the 

 wheel until the movable line traverses one space on the stage mi- 

 crometer. Each tooth of the recording comb indicates a total 

 revolution of the wheel, and by noting the number of teeth required 

 and the graduations on the wheel, the revolutions and part of a 

 revolution required to measure the y-^ mm. of the stage micrometer 



FIG. 119. Filar Micrometer Ocular. This filar micrometer ocular is of 

 the Ramsden type and consists of a positive ocular with a moveable hair line 

 and two reference lines at right angles to each other as shown in A. The 

 moveable line must be carried over the entire length of the object to be meas- 

 ured by rotating the drum. 



A. Field of the filar micrometer showing the moveable and the cross 

 lines, and the comb. The teeth serve to measure the total revolutions of the 

 drum. (Cut loaned by the Bausch & Lomb Optical Co. ) 



can be easily noted. Measure in like manner 4 or 5 spaces and get 

 the average. Suppose this average is i^ revolutions or 125 grad- 

 uations on the wheel, to measure the T | mm. or io// (see 182), 

 then one of the graduations on the wheel would measure 10/1 divided 

 by 125 .o8/*. In using this valuation for actual measurement, the 

 tube of the microscope and the objective must be exactly as when 

 obtaining the valuation (see 187, 194). 



