4
Align the substage condenser by
centering the image of the field
diaphragm, using the condenser’s radial
centering screws (
21
). If necessary,
refocus the condenser once again, to
keep the field diaphragm in sharp focus
with the specimen image.
5
Open the field diaphragm until it is just
outside the field of view or, for
photography, until it is just beyond the
area of the film format (
22
). Do not open
it any farther, since this could cause flare
and a loss in contrast.
Adjusting the aperture diaphragm
The simplest method of adjustment is to
close the aperture only far enough to
notice a loss of brightness. A more
precise method of adjustment is to
remove the eyepiece (or camera) and
look down the tube. The correct setting
will generally be such that the circle of
light within the diaphragm blades has a
diameter about 2/3 to 3/4 that of the entire
light disc (
23
).
The proper setting is critical for good
photomicrographs; the diaphragm should
be closed down sufficiently to provide the
desired image contrast, but not so far as
to cause unwanted diffraction patterns
that result in a loss of detail.
The light that forms the microscope
image may be either direct (absorbed) or
deflected (refracted/diffracted). The aper-
ture diaphragm controls the “mix” of
direct and deflected light. The correct set-
ting is somewhat dependent on the type
of specimen, and how that specimen uses
direct and deflected light. For example:
•
A diatom uses mainly deflected light, and
not much direct light. The aperture
diaphragm should be closed down to
about 25%, to produce the best image.
•
Stained tissue uses both direct and
deflected light; an aperture diaphragm of
50% to 90% will produce a good image.
•
A stained blood smear uses mainly direct
light, very little deflected light. An
aperture diaphragm setting anywhere
between 25% and 90% will produce a
good image.
English 11
