270
8.
MICROSCOPY
sectional diameter of the main beam is particularly small, and apertures
of 20-50 [x can and should be used, at least in RCA microscopes with
single condenser lenses. The smaller the aperture, the better the final
contrast, but one pays for this in that the contamination rate is increased
and astigmatism becomes a very serious problem. To avoid asymmetrical
contamination, the intense cross-over beam must never be allowed to
touch the rim of the aperture. Apertures approximately 35 \i in diameter
are a reasonable compromise, being large enough to accommodate the
cross-over beam with a small margin of safety, yet providing good contrast.
Between-the-lens apertures are fairly close to the specimen. They are
quite vulnerable to materials which may evaporate from this, most
particularly the decomposition products of embedding plastics. Thus,
they tend to contaminate most rapidly in this position. Apertures in the
back focal plane of the lens are much further removed from the specimen
and their contamination rate is slower. The cross-over beam is wider
here, however, than in the between-the-lens position, so that the apertures
must be correspondingly larger. Apertures of 50 \x in the back focal plane
correspond approximately to 35 \i apertures in the other position (Fig. 30).
If an aperture is used in the back focal plane, it is important that it
really be near the level of this plane. If it is either too close or too far
from the lens it will restrict the optical pathway so that the edges of the
aperture will be seen in silhouette in the final image. Even more importantly, however, its rim will be bombarded unnecessarily heavily by a
nearly cross-over beam, and it will be impossible to avoid astigmatism
for long.
The position of the back focal plane depends upon the focal length of
the lens. Unlike glass lenses which have fixed focal lengths, magnetic
lenses, of course, are varied at will simply by altering the current flow.
It is necessary that the aperture be in the back focal plane when the lens
system is operating in a manner to focus a specimen. In getting ready to
find this position, a specimen first is focused at a fairly high magnification, and the position of all of the lens control knobs noted. The height
of the aperture is adjusted to correspond with this particular focal length.
To find the back focal plane, one takes advantage of the fact that the
aperture edge can be seen in silhouette except when the aperture is in the
proper position. One starts with an old aperture in the holder, and
deliberately moves its rim into the beam so that it can be seen as a somewhat out-of-focus silhouette on the viewing screen. The operator will not
8.
MICROSCOPY
sectional diameter of the main beam is particularly small, and apertures
of 20-50 [x can and should be used, at least in RCA microscopes with
single condenser lenses. The smaller the aperture, the better the final
contrast, but one pays for this in that the contamination rate is increased
and astigmatism becomes a very serious problem. To avoid asymmetrical
contamination, the intense cross-over beam must never be allowed to
touch the rim of the aperture. Apertures approximately 35 \i in diameter
are a reasonable compromise, being large enough to accommodate the
cross-over beam with a small margin of safety, yet providing good contrast.
Between-the-lens apertures are fairly close to the specimen. They are
quite vulnerable to materials which may evaporate from this, most
particularly the decomposition products of embedding plastics. Thus,
they tend to contaminate most rapidly in this position. Apertures in the
back focal plane of the lens are much further removed from the specimen
and their contamination rate is slower. The cross-over beam is wider
here, however, than in the between-the-lens position, so that the apertures
must be correspondingly larger. Apertures of 50 \x in the back focal plane
correspond approximately to 35 \i apertures in the other position (Fig. 30).
If an aperture is used in the back focal plane, it is important that it
really be near the level of this plane. If it is either too close or too far
from the lens it will restrict the optical pathway so that the edges of the
aperture will be seen in silhouette in the final image. Even more importantly, however, its rim will be bombarded unnecessarily heavily by a
nearly cross-over beam, and it will be impossible to avoid astigmatism
for long.
The position of the back focal plane depends upon the focal length of
the lens. Unlike glass lenses which have fixed focal lengths, magnetic
lenses, of course, are varied at will simply by altering the current flow.
It is necessary that the aperture be in the back focal plane when the lens
system is operating in a manner to focus a specimen. In getting ready to
find this position, a specimen first is focused at a fairly high magnification, and the position of all of the lens control knobs noted. The height
of the aperture is adjusted to correspond with this particular focal length.
To find the back focal plane, one takes advantage of the fact that the
aperture edge can be seen in silhouette except when the aperture is in the
proper position. One starts with an old aperture in the holder, and
deliberately moves its rim into the beam so that it can be seen as a somewhat out-of-focus silhouette on the viewing screen. The operator will not