282
8.
MICROSCOPY
important at low and moderate magnifications, but it becomes increasingly serious as one strives for high resolution.
A fairly good alignment is necessary just as a matter of convenience.
In the aligned microscope one can change magnifications, vary illumination, make large focus changes, and so forth, all without having to readjust the source of illumination much, or recenter the image. In this
practical sense, the alignment of the condenser aperture and the intermediate lens are most critical. Fortunately their alignment is quite simple.
Full column alignment involves all of the following elements: (1) the
electron source (filament tip); (2) the anode; (3) the condenser lens (sometimes consisting of two independent lenses); (4) the objective lens; (5) the
intermediate lens; (6) the projector lens; (7) the viewing screen and
photographic plate. Furthermore, it has already been stressed that the
histologist will have a movable aperture in the objective lens, and he
may very well have one in the condenser lens system. Each of these
parts must be aligned with its immediate neighbors. In a very few
simplified electron microscopes one or two of these parts may be missing.
In some other instruments certain of these components may be coupled
together. Thus, usually, the projector lens and the viewing screen are
coupled. In the RCA EMU-2 microscopes, the intermediate lens is
coupled with the projector lens. It is usual to have most apertures fixed
in an instrument excepting those in the objective lens and perhaps in the
condenser lens. Coupled elements actually represent compromises with
true alignment. The next sections consider the particular problems of
the different movable elements.
8.7. Objective Alignment
The objective lens is aligned with the projector lens. Two types of
alignment actually are possible. The one which originally was recommended is termed "current alignment." Because electrons spiral through
a magnetic lens, if the focusing current is varied, the image is seen to
spiral about an optical axis. For current alignment, the objective lens
was moved about on top of the projector lens until this optical axis coincided with the center of the viewing screen. Unfortunately this axis
will not necessarily correspond very well with the axis observed when
electrons of differing speeds pass through the lens. The latter is termed
the axis of "voltage alignment." For reasons discussed in the previous
8.
MICROSCOPY
important at low and moderate magnifications, but it becomes increasingly serious as one strives for high resolution.
A fairly good alignment is necessary just as a matter of convenience.
In the aligned microscope one can change magnifications, vary illumination, make large focus changes, and so forth, all without having to readjust the source of illumination much, or recenter the image. In this
practical sense, the alignment of the condenser aperture and the intermediate lens are most critical. Fortunately their alignment is quite simple.
Full column alignment involves all of the following elements: (1) the
electron source (filament tip); (2) the anode; (3) the condenser lens (sometimes consisting of two independent lenses); (4) the objective lens; (5) the
intermediate lens; (6) the projector lens; (7) the viewing screen and
photographic plate. Furthermore, it has already been stressed that the
histologist will have a movable aperture in the objective lens, and he
may very well have one in the condenser lens system. Each of these
parts must be aligned with its immediate neighbors. In a very few
simplified electron microscopes one or two of these parts may be missing.
In some other instruments certain of these components may be coupled
together. Thus, usually, the projector lens and the viewing screen are
coupled. In the RCA EMU-2 microscopes, the intermediate lens is
coupled with the projector lens. It is usual to have most apertures fixed
in an instrument excepting those in the objective lens and perhaps in the
condenser lens. Coupled elements actually represent compromises with
true alignment. The next sections consider the particular problems of
the different movable elements.
8.7. Objective Alignment
The objective lens is aligned with the projector lens. Two types of
alignment actually are possible. The one which originally was recommended is termed "current alignment." Because electrons spiral through
a magnetic lens, if the focusing current is varied, the image is seen to
spiral about an optical axis. For current alignment, the objective lens
was moved about on top of the projector lens until this optical axis coincided with the center of the viewing screen. Unfortunately this axis
will not necessarily correspond very well with the axis observed when
electrons of differing speeds pass through the lens. The latter is termed
the axis of "voltage alignment." For reasons discussed in the previous