152 ELEMENTARY CHEMICAL MICROSCOPY 



of non-variable magnitude, but the determination of the ocular 

 micrometer value may be made more exact by eliminating 

 fractions as shown in Fig. 94,* where it is evident that a whole 

 number of ocular scale divisions are included in a whole number 

 of divisions of the stage micrometer. This is accomplished by 

 altering the ratio between the images of the two scales through a 

 change in the position of the draw-tube. Start with the draw- 

 tube pushed in as far as it will go. Pull it out one or two milli- 

 meters at a time, lining up and focusing the stage scale each 

 time, until one or more spaces of the stage scale are covered by a 

 whole number of divisions of the ocular scale. With the class 

 of objectives commonly employed of comparatively low powers, 

 the use of a tube length slightly different from that for which 

 the lenses are designed, effects their resolving power so little as 

 to be negligible. In order that the conditions may be duplicated 

 under which the ocular micrometer value has been obtained, it 

 is obvious that a record must be made of the draw-tube length 

 employed; the notebook entry will, therefore, take some such 

 form as this: 



1 6 millimeter objective, draw- tube 175; i division ocular scale = 

 o.oi millimeter = 10 /*. 



When high power objectives are employed the rulings of the 

 stage micrometer will appear as very thick or coarse lines. It 

 then becomes essential to observe special precautions in the 

 adjusting of the ocular and stage scales, for if the adjustment 

 shown in Fig. 95 C were to be followed, it is evident that an error 

 will be introduced equal to at least half the thickness of the 

 coarse stage rulings. Either the ocular micrometer scale lines 

 must be placed at the center of the coarser stage lines, as shown 

 in A, or the ocular lines may be placed at the right or left edges 

 of the stage lines, but always all of them on the same sides as shown 



1 Figs. 93 and 94 were drawn by means of a camera lucida and therefore show 

 exactly the conditions met with. Each division on the stage micrometer (the 

 lines crossing the entire field) equals o.i mm. With a pair of dividers compare 

 the magnitude of a space in Fig. 93 with one in Fig. 94. It will be found that 

 lengthening the draw-tube has changed the ratio between images of stage and 

 ocular scales. 



