Figure H represents a fine adjustment enscmble quite
similar to that of Figure G except that the micrometer
thread is replaced by a worm-gear device. A segment
of a gear takes the place of the nut and the long arm
of the bell crank lever. The impulse from the short
arm is upward, producing no lateral thrust as is also the
case in Figures F and G. On all of the above described
adjustments provision is made for automatically stop-
ping the revolution of the shaft when the adjustment
has reached the limit of its excursion, thus avoiding
any injury to the threads or gears.
Figure I represents another worm-gear fine adjustment
in which the complete gear circle is used. On this gear
is fastened an eccentric heart shaped cam on the peri-
phery of which a small roller is made to revolve. This
roller is attached to the movable parts of the fine
adjustment. When the gear levolves in the direction
of the arrow as shown in the cut, one of the components
of the movement of the heart shaped cam forces the
roller upward, and will continuc to do so uritil the apex
of the heart passcs undcr thc roller whcn it will then
move downward for 180" of the revolution. It has the
advantage thaL the fine adjustmcnt never "runs out."
An entircly ncw fine atljustment (see Figure J) has
recently come out.in which_ the necessary reduction
,FB is attained by means of a chain of
splrr gears similar to clock gears
working in conjunction with a lever.
It is very different from the types
just described. It provides the
dircct upward thrust. The fine ad-
justment head is represented at F.B.
All standard microscopes are now
made so that the positive impulse is always upward
l4
against a slightly compressed spring. The mechanism
allows the movable parts to descend by gravity and
the force of the compression spring. Therefore in focus-
sing down one is not so apt to cause damage if the
front of the objective should come into contact with
the cover glass. Avoid this contact if possible. If at all
uncertain observe the proximity of the objective to
the cover glass before looking into the eyepiece.
ff when looking into the eyepiece no change of focus
is noticed by turning the fine adjustment, it is quite
possible that the objective is resting on the cover glass.
It is possible that the fine adjustment may have " run
out." It is always bcst to kecp the finc adjustment
about midr,vay in its range. If the microscope is one
provided with positivc stops :rt the ends of its cxcursion,
it will be necessary to place the rnechanism sornewhere,
near the midway point. If the microscope is one with
the fine adjustment head at the top of the arm, the
loose pin in the end of the micrometer screw may have
been misplaced. See page 12.
The Coarse Adjustment
The Coarse Adjustment or rapid movement of the body
tube is now affected on all microscopes by means of
the diagonal rack and pinion. The bearing surfaces on
the American and lluropean instruments are quite dif-
ferent. Both are good. 'Ihere is no advantage either
way except for the oil grooves on Spencer microscopes.
The bearings are very closely fitted. Any foreign matter
on the surfaces scriously interferes.
Do not strain the teeth of the rack and pinion by forcing
the bearings llack and forth over one another when they
are not clean. A little xylol or chloroform rubbed on
the surface will clean them. Do not use emery o. ?;r"
Fis. J
