184 



ELEMENTARY CHEMICAL MICROSCOPY 



increased the magnitude of the distance of the extreme ocular 

 line from the scale division on the stage micrometer nearest to 

 it, then instead of shortening the draw- tube, the draw- tube 

 should be extended. 



In order to expedite future measurements it is always advis- 

 able to try and obtain such a position of the draw-tube as will 

 yield the least cumbersome value possible in the ratios of stage 

 to ocular scale divisions. 



With the class of objectives commonly employed of com- 

 paratively 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 va'ue 

 has been obtained, it is obvious that a record must be made of 

 the draw-tube length employed; the notebook entry will, there- 

 fore, take some such form as this: 



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

 0.01 millimeter = 10 /jl. 



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. 117 C were to be followed, it is evident that an 



Incorrect 



Correct Correct 



Fig. 117. Determining the Ocular Micrometer Ratio: Heavy Lines 

 Micrometer, Light Lines = Ocular Micrometer. 



Stage 



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 



