284
8.
MICROSCOPY
indeed, have to be if the pole-piece is apertured. Microscopists whose
work demands that they strive for maximum resolution may feel that
more frequent alignment checks are necessary, but most work with sections will not be handicapped by a minor objective lens misalignment.
After the objective and projector lenses have been aligned, the intermediate lens (if not coupled to the projector lens) is added functionally
to the system. In most microscopes this is used as a reducing lens to extend the lower ranges of magnification. For this purpose one wants
alignment for operational convenience. One turns the magnification
control in its lower ranges and observes the center of rotation of the
image. Once again this indicates the optical axis of the lens, and the lens
is moved about until this coincides with the center of the viewing screen
(current alignment).
In some microscopes the intermediate lens is used additively at high
magnifications. If one regularly uses this end of the magnification scale,
it may be desirable to align the intermediate lens for this, rather than for
low magnifications, since it is not likely that one alignment will do very
well for both.
8.8. Alignment of the Illuminating System
The previous section dealt with the alignment of those lenses which
are involved in the magnification and projection of the image. Obviously
this presupposes that one has suitable illumination reaching the specimen
and the objective lens. This represents a rather separate problem. There
are at least three elements to this system in all but a few simplified microscopes. The filament is the actual source of electrons, of course. The latter
are given acceleration and direction by the applied voltage, and the shape
and positioning of the hole in the anode cap. Then the condenser lens
allows this beam to be modified convergently or divergently. When the
beam leaves the condenser lens its axis must nearly coincide with the
optical axis of the objective lens.
Two basic steps are necessary to align the illuminating system. The
first may be spoken of as "source alignment," which is concerned with
the alignment of filament tip, anode, and optical axis of the objective
lens. In essence the gun must be tipped so that its optical axis is parallel
with that of the objective lens. The degree of tilt can be varied in most
microscopes, either by actual mechanical tipping, or by controllable
8.
MICROSCOPY
indeed, have to be if the pole-piece is apertured. Microscopists whose
work demands that they strive for maximum resolution may feel that
more frequent alignment checks are necessary, but most work with sections will not be handicapped by a minor objective lens misalignment.
After the objective and projector lenses have been aligned, the intermediate lens (if not coupled to the projector lens) is added functionally
to the system. In most microscopes this is used as a reducing lens to extend the lower ranges of magnification. For this purpose one wants
alignment for operational convenience. One turns the magnification
control in its lower ranges and observes the center of rotation of the
image. Once again this indicates the optical axis of the lens, and the lens
is moved about until this coincides with the center of the viewing screen
(current alignment).
In some microscopes the intermediate lens is used additively at high
magnifications. If one regularly uses this end of the magnification scale,
it may be desirable to align the intermediate lens for this, rather than for
low magnifications, since it is not likely that one alignment will do very
well for both.
8.8. Alignment of the Illuminating System
The previous section dealt with the alignment of those lenses which
are involved in the magnification and projection of the image. Obviously
this presupposes that one has suitable illumination reaching the specimen
and the objective lens. This represents a rather separate problem. There
are at least three elements to this system in all but a few simplified microscopes. The filament is the actual source of electrons, of course. The latter
are given acceleration and direction by the applied voltage, and the shape
and positioning of the hole in the anode cap. Then the condenser lens
allows this beam to be modified convergently or divergently. When the
beam leaves the condenser lens its axis must nearly coincide with the
optical axis of the objective lens.
Two basic steps are necessary to align the illuminating system. The
first may be spoken of as "source alignment," which is concerned with
the alignment of filament tip, anode, and optical axis of the objective
lens. In essence the gun must be tipped so that its optical axis is parallel
with that of the objective lens. The degree of tilt can be varied in most
microscopes, either by actual mechanical tipping, or by controllable