42 
IVIECIIANICAL PORTION OF THE MICROSCOPE. 
especiall}^ to facilitate the adjustment of the draw-tube, when 
using the micrometer. 
59. Rules for Adjusting: Object-Olasses. As many per- 
sons who use the miscroscope find some embarrassment in accu- 
rately adjusting the object-glass for the thickness of the cover 
on different objects, and hence fail to appreciate the real excel- 
lence of the instrument, we subjoin a few simple rules, by 
means of which any one may soon learn to make this cor- 
rection. 
Select any dark speck or opaque portion of the object, and 
bring the outline into perfect focus ; then lay the finger on the 
milled head of the fine motion, and move it briskly backwards 
and forwards in both directions from the first position. Ob- 
serve the expansion of the dark outline of the object, both 
when within and when without the focus. If the greater ex- 
pansion, or coma, is when the object is within the focus, or 
nearest to the objective, the lenses must be placed farther 
apart, or towards the mark uncovered P If the greater 
coma is when the object is without the focus, or farthest from 
the objective, the lenses must be brought closer together, or to- 
wards the mark ‘Hhin glassP When the object-glass is in 
proper adjustment, the expansion of the outline is exactly the 
same both within and without the focus. 
A different indication, however, is afforded by such test ob- 
jects as present (like the Podura scale and Diatomaceje) a set 
of distinct dots or other markings. For if the dots have a 
tendency to run into lines when the object is placed within 
the focus, the glasses must be brought closer together ; on the 
contrary, if the lines appear when the object is without the 
focal point, the combinations of the object-glass must be fur- 
ther separated. 
60. Delicacy of tlie Adjii§tmciit for Tliiot Covers. 
When the angle of aperture is very wide, the difference in 
the aspect of any severe test, under different adjustments, 
becomes at once evident; markings which are very distinct 
when the correction has been exactly made, disappearing al- 
most instantaneously when the graduated collar has been turned 
J. & W. GRUNOW & CO’S ILLUSTRATED 
