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
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