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14
Use
of the field diaphlllgm
Ifoptimum
contrast
is
to
be
attained,
it
is
important
to
i.llumin
ate
only
tb
e
part
of
the
specimen that appears
in
the
image.
Therefore
th
e
fi eld
diaphragm is
only opened as
far
as
the field
of
view.
For this reason, a change
of
magnification
always
necessitates
matching the field
diaphragm to
the
object
field.
Use of the aperture diapbragm
The more
tbe aperture diaphragm is
narrowed. the higher
the
image
contras
t Axial resolving power
(image
sha
rpn
ess
in
the
direction of
the optical axis) also
increases.
However,
exces
~ive
closure
of the
aperture
diaphragm
has
an adverse effect on
la teral
resolving power (selectivity).
The
best
optical
pe
rformance is
ob
tained
when
the
apertures
of
th
e
objecti
ve
and
the
condense
r
are
the
sa me.
A
visible
deterioration of
resolving
power
is
seen wben
tbe
aperture
dia pbrag
m is closed
over
'13 of
the
aperture of the
objecti
ve. a
nd
tbis s
bould
be
avoided
as
far
as
possible. To
c
heck
this. an
eyepiece must
be
taken
out
of
the
eyepiece
tube,
the aperture
diaphragm narrowed until it
s image cove rs
1
13
of
the illuminated rear
lens area. Replace
the
eyepiece.
If
necessary,
the aperture diaphragm
can
be
na rrowed
further
for
objects
with weak contrast.
Once
determined. the aperture
diaphragm
setting can
be
reproduced with the aid of the
scales
on
the brightfield and
phase contrast
condensers.
Note:
The
aperture
diaphragm should
not
be
used to
set image
inten
sity.
This is
done
with the rotary
knob
(12.2) for
reg ulat
ion
of
lamp
intensity, or
for
photomicrography,
with neutral
light
blocking filters.
Oil immersion objectives
Oil
immersioo
objectives
are labe
ll
ed
with
th
e
additional
e
ngra ving
"OEL"
and
a black ring
on
the
lower
edge of
the
objective mount.
The
y
must
always
be
used
with immersion
oil.
Before
the oil immersion
objec tive
is
screwed
into
the
nosepiece a drop
of immersion
oil
must be
applied
to
the
speci
men. Check that the
immersion oil
is
free
from air
bubbles.
The immersion oil
has
almost the
same refrac
tive index ne=
1.515 as the
coverglass
and
the
front
lens of
the microscope
objective. The focal length
and
th
e
working distance of an
immersion objective
are
usually ve ry
small. For this reason,
working with oil
immers ions requi res great ca
re.
For routine work, the
ord
in
ary 0.90 condenser
is adequate
even
for oil immersions. However,
if
the
full
aperture
of the
immer
sion
objective
is
to
be
used, e.g. for
very fine structures,
the
co
ndenser
aperture
must
also
be
enlarged
.
For
co
nden ser no.
56
this
ca
n be
done
by sc
rewing
o
n
a
1.2
5
OEL
co ndense
r
ca
p,
o
r for
the
LK
co
nd
e nse
r,
a
n
APL
OEL
1.32
S
1.1
co
ndenser
top.
In
these cases,
imm
e rsio
n
o
il must
also be a pplied be
tween
the
condense
r
cap/top
and tbe
undernea
th of the specimeo
slide.
After the
inves
tigation,
aU
surfaces
where
immersion
oil
has
been applied must be
carefu lly cleaned.
U
se
a
soft
clo
th mois
tened
with alcohol
o
r
benzine.
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