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 value 
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: 
i6 millimeter objective, draw-tube 175; i division ocular scale = 
o.oi millimeter = 10 /x. 
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 
A B 
Correct Correct Incorrect 
Fig. 117. Determining the Ocular Micrometer Ratio: Heavy Lines = Stage 
Micrometer, Light Lines = Ocular Micrometer. 
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 
