148 



MEASURING WITH THE MICROSCOPE 



[Ch. V 



For obtaining the valuation of this ocular micrometer proceed as 

 follows: Employ a 4 mm. objective. Carefully focus the TTir mm. 

 lines. The lines of the ocular micrometer should also be sharp; 

 if they are not focus them by moving the ocular up or down in the 

 shding tube. Make the vertical lines of the ocular micrometer parallel 

 with the lines of the stage micrometer (fig. 90, 98). Note the posi- 

 tion of the graduated drum and the teeth of the recording comb, and 

 then rotate the wheel until the movable line traverses one space on 



the stage microm- 

 eter. Each tooth 

 of the recording 

 comb indicates a 

 total revolution of 

 the wheel, and by 

 noting the number 

 of teeth reqizired 

 and the gradua- 

 tions on the wheel, 

 the revolutions and 

 part of a revolu- 

 tion required to 

 measure the o.oi 

 mm. of the stage 

 micrometer can 

 be easily noted. 

 Measure in like manner 4 or 5 spaces and get the average. Suppose 

 this average is ij revolutions or 125 graduations on the wheel, to 

 measure the o.oi mm. or 10 ju, (see § 246), then one of the graduations 

 on the wheel would measure 10 ju. divided by 125 = 0.08 ju,. In using 

 this valuation for actual measurement, the tube of the microscope 

 and the objective must be exactly as when obtaining the valuation 

 (see § 235-242). 



The valuation of the filar micrometer can be obtained for any 

 objective by proceeding exactly as above (see § 252 for measure- 

 ment). 



Fig. 93. Filar Micrometer Ocular. 



(From the i6th ed. of the Catalogue of the Bausch & 

 Lomb Optical Co.). 



This is a Ramsden ocular, and the recording drum is 

 divided into 100 equal divisions, and as the pitch of the 

 screw is 0.5 mm., each division on the drum represents 

 an actual movement of 0.005 mm. of the movable line. 



