14
Use
of the field diaphlllgm
I
fop
timum
contrast is to be attained,
it
is important to i.llumin-
ate only tbe part
of
the
specimen
that
appears in the image.
Therefore the
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 diaphr
agm
to
the
ob
ject field.
U
se
of the aperture diapbragm
The
more
tbe
aperture
diaphragm
is
narrowed.
the higher the
image
contras
t Axial resolving power (image sharpness
in
the
direction
of
the
optical axis) also
in
creases. However, exces-
~ive
closure
of
the
aperture
diaphragm
has
an
adverse effect
on
lateral resolving
power
(selectivity).
The best optical pe
rformance
is
ob
tained
when
the ape
rtur
es
of
th
e objec
ti
ve
and the condenser are the
sa
me.
A visible deterioration
of
r
eso
lv
in
g power
is
see
n
wb
en
tb
e
aperture dia
pbr
agm is closed
over
'13
of
the ap
ertur
e
of
the
objec
ti
ve. and tbis sbould be avoided
as
far as possible. To
check this.
an
eyep
iece
must
be taken
out
of
the
ey
epie
ce
tube
,
th
e
ape
rture diaphragm narrowed until its im
age
covers 1
13
of
the illuminated rear lens area.
Replace
the eyepiece.
If necessary,
the
aperture
diaphragm
can be na
rr
owed
further
f
or
objects with weak contrast.
Once
determined.
the
aperture
diaphragm
s
ettin
g
can
be
reproduced with the aid
of
the
scales on the brightfield
and
phase contrast condensers.
Note:
Th
e
aperture
diaphragm
should
not
be used to set image inten-
sit
y.
This
is
done
with
the
rotary
kn
ob (12.2) for regul
at
ion
of
lamp intensity,
or
for
photomi
crography, with neutral light-
blocking filters.
Oil
immersion objectives
Oil immersioo objectives are labe
ll
ed with
th
e additional
engraving "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
imm
ersion oil
is
free
from
air
bubbles.
The
immersion oil has almost the
same
refractive index
ne=
1.515
as the coverglass
and
the
front
l
ens
of
the
microscope
objective.
The
focal
length
and
the working
distance
of
an
immersion objective are usually very small. For this reason,
working with oil immersi
ons
requi res great
ca
re.
For routine work, the ordin
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
nd
ense
r aperture must also be enlarged. For
co
ndenser no.
56
this can be
done
by scr
ew
ing on a
1.2
5
OEL
co
nden
ser cap,
or for the LK conde nser, an
APL
OE
L
1.32
S
1.1
co
nden
ser top.
In
the
se
cases, immersion o
il
must al
so
be applied between
th
e
con
den
ser
cap/top
an
d
tbe
underne
ath of
the
sp
ec
imeo
slide.
After the investigation, aU surf
aces
where immersion oil has
been
applied
must
be carefu
ll
y cl
eaned.
Use a
so
ft
cloth mois-
tened
with
alcohol or
ben
zine.
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