Microscope parts (17)
a
Phototube eyepiece
b
Phototube
c
Viewing eyepiece
d
Revolving nosepiece
e
Objective lens
f
Specimen
g
Specimen stage
h
Substage condenser
i
Aperture diaphragm adjustment
j
Condenser centering screws
k
Field diaphragm adjustment
l
Field diaphragm
m
Condenser focusing knob
n
Coarse and fine focusing knobs
o
Lamp
p
Lamp voltage control
Filter placement
Filters should be placed in the filter
holder (
18-q
) if the microscope has one.
Alternatively, they can be placed where
the light exits from the microscope base
(
18-r
). If the latter location is used, be
sure the filters are clean and undamaged.
The nearer they are to the field
diaphragm, the more likely any blemishes
on the filter will be focused with the
specimen.
The addition of certain color filters may
change the effective focus of the
microscope (depending on the optical
characteristics of the objective lens). After
adding filters, re-focus the image before
taking pictures.
Koehler illumination
Most modern compound microscopes are
designed for Koehler illumination, which
is ideal for photomicrography. This
technique involves proper alignment of
the light source, alignment and focus of
the substage condenser (
17-h
) and field
diaphragm (
17-l
), and adjustment of the
aperture diaphragm (
17-i
). Koehler
illumination provides even lighting and,
in addition, enables the objective lens to
produce optimum resolution.
Alignment and focus of the substage
condenser and the field diaphragm
1
Place a specimen on the stage and focus
it accurately. Slowly close the aperture
diaphragm of the substage condenser to
a point where you see a distinct reduction
in brightness through the eyepiece. (See
also
Adjusting the aperture diaphragm.)
2
Close the field diaphragm to its smallest
setting. Viewing through the eyepiece,
raise or lower the substage condenser
until the edge of the field diaphragm is
sharply focused with the specimen image
(
19
).
3
Open the field diaphragm to about 3/4 of
the visual field and refocus the edge of
the diaphragm, as above (
20
).
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