8.12.
OBJECTIVE LENS COMPENSATION
291
and deteriorate images seriously. It is wise to check for astigmatism daily
as one begins operation in the morning. Unless one has extreme confidence in his colleagues he should also check this after anyone else has
used the microscope. When astigmatism appears it is essential that it be
eliminated by compensation if it is not so severe as to require a new
aperture. It is a "must" that you learn to use an externally controlled
"stigmator" which should be considered mandatory equipment nowadays in any microscope used for section work. Either electrostatic compensation or its magnetic equivalent is acceptable. This is a feature of
most new microscopes and is available as an accessory (manufactured by
the Canal Industrial Corporation of Bethesda, Maryland) for the RCA
EMU-2 and EML instruments. The principle of the electrostatic method
is described in Chapter 8.1.
FIG. 34.
(a)-(c) Overfocused, focused, and underfocused micrographs, respectively,
of a complex dirt particle. The focused image shows much pattern complexity and a
long range of tonal values. The out-of-focus images lack complexity and tonal variation.
(d)-(h) A through-focus series of micrographs of a small hole in a film at high magnification; (d) shows the diffraction pattern of an overfocused image with a characteristic
apparent light band outlined by black; (e) is very close to true focus with just a suggestion of overfocusing; (f) also is very near true focus but shows a slight rim of white
suggestive of underfocusing. The bright band is more conspicuous in (g) which is
definitely underfocused. An edge such as this has much greater apparent contrast than
that of (e) and the novice is apt to think that this represents true focus when this is
not so. In (h) the bright diffraction band of the decidedly underfocused edge is conspicuous and unmistakable. These diffraction bands are known as Fresnel fringes.
Figure 40 demonstrates the effect of focal level, and hence diffraction, on the image
observed.
The micrographs of (d)-(h) were made with a well compensated pole-piece using electrostatic compensation to counterbalance residual astigmatism, (j) shows the intrinsic astigmatism that existed in the lens system before compensation, (i) indicates how a relatively
large, asymmetric electrostatic field can be superimposed, and its axis oriented at right
angles to the axis of the original astigmatism. (The broken lines represent the axes of intrinsic astigmatism, the solid lines, axes of superimposed electrostatic force.) By reducing
the intensity of the superimposed field, an exact balance of the opposing forces can be
achieved to yield the compensated images of (d)-(h). In (k) the electrostatic force
required to achieve compensation [as in (d)] was superimposed upon the axis of original
astigmatism to produce an additive effect when compared with (j). (1) is the overfocused
image of a hole in a film which was drifting when the photographic exposure was made,
(m) is the overfocused image of a hole in a microscope showing "directional softness"
(in this case due to an oscillating instability in the high voltage supply). Notice that
the images of (1) and (m) are blurred at the poles of the axes of movement rather than
being in-focus as are the poles of one axis of (i)-(k).
OBJECTIVE LENS COMPENSATION
291
and deteriorate images seriously. It is wise to check for astigmatism daily
as one begins operation in the morning. Unless one has extreme confidence in his colleagues he should also check this after anyone else has
used the microscope. When astigmatism appears it is essential that it be
eliminated by compensation if it is not so severe as to require a new
aperture. It is a "must" that you learn to use an externally controlled
"stigmator" which should be considered mandatory equipment nowadays in any microscope used for section work. Either electrostatic compensation or its magnetic equivalent is acceptable. This is a feature of
most new microscopes and is available as an accessory (manufactured by
the Canal Industrial Corporation of Bethesda, Maryland) for the RCA
EMU-2 and EML instruments. The principle of the electrostatic method
is described in Chapter 8.1.
FIG. 34.
(a)-(c) Overfocused, focused, and underfocused micrographs, respectively,
of a complex dirt particle. The focused image shows much pattern complexity and a
long range of tonal values. The out-of-focus images lack complexity and tonal variation.
(d)-(h) A through-focus series of micrographs of a small hole in a film at high magnification; (d) shows the diffraction pattern of an overfocused image with a characteristic
apparent light band outlined by black; (e) is very close to true focus with just a suggestion of overfocusing; (f) also is very near true focus but shows a slight rim of white
suggestive of underfocusing. The bright band is more conspicuous in (g) which is
definitely underfocused. An edge such as this has much greater apparent contrast than
that of (e) and the novice is apt to think that this represents true focus when this is
not so. In (h) the bright diffraction band of the decidedly underfocused edge is conspicuous and unmistakable. These diffraction bands are known as Fresnel fringes.
Figure 40 demonstrates the effect of focal level, and hence diffraction, on the image
observed.
The micrographs of (d)-(h) were made with a well compensated pole-piece using electrostatic compensation to counterbalance residual astigmatism, (j) shows the intrinsic astigmatism that existed in the lens system before compensation, (i) indicates how a relatively
large, asymmetric electrostatic field can be superimposed, and its axis oriented at right
angles to the axis of the original astigmatism. (The broken lines represent the axes of intrinsic astigmatism, the solid lines, axes of superimposed electrostatic force.) By reducing
the intensity of the superimposed field, an exact balance of the opposing forces can be
achieved to yield the compensated images of (d)-(h). In (k) the electrostatic force
required to achieve compensation [as in (d)] was superimposed upon the axis of original
astigmatism to produce an additive effect when compared with (j). (1) is the overfocused
image of a hole in a film which was drifting when the photographic exposure was made,
(m) is the overfocused image of a hole in a microscope showing "directional softness"
(in this case due to an oscillating instability in the high voltage supply). Notice that
the images of (1) and (m) are blurred at the poles of the axes of movement rather than
being in-focus as are the poles of one axis of (i)-(k).