9.5.
REASONS FOR UNSATISFACTORY PICTURES
309
only when he has completed the finished prints. In assembling the following summary, some effort has been made to arrange these difficulties in
order of frequency.
(1) Unduly thick sections. The beginner certainly will be tempted to
take pictures of sections that are too thick to show good resolution (Fig.
36). This is easily corrected with experience when the operator learns
to equate properly the contrast values of the fluorescent screen with those
that will appear on the photographic plate.
(2) Unduly thick supporting films. Any supporting film reduces resolution and contrast so that the only good film is one which barely sustains
the specimen. The operator should be willing to sacrifice many potential
grid squares in order to achieve quality in the surviving ones. The beginner should be warned that a fairly thick section on a thick supporting film
may look deceptively good on the fluorescent screen. It will have most of
the contrast properties of a thin section on a thin film, but will, of course,
lack the resolution of the latter.
(3) Specimen drift during picture taking. This was formerly a very
important source of trouble. It need not be serious now if cross-linked
plastic embedments are employed, if carbon films or carbon stabilized
films are used in conjunction with fairly rapid shutter speeds, and if
severely torn specimens are avoided.
(4) Changes in focus during preparations for photography. The beginner is apt to feel that he needs an unduly bright beam for accurate
focusing. If the operator then reduces the intensity greatly, there may be
a change in the focal plane of the objective lens so that the image he
photographs will be, in fact, out of focus. There is less danger of this
now than formerly because pictures are taken much more rapidly than
originally, but every operator should carefully explore and master the
characteristics of his instrument, and learn what the tolerances may be.
(5) Objective lens astigmatism. This limits the possibilities of obtaining high resolution pictures. It can be guarded against by making a daily
practice of checking image quality (Chapter 8.12).
(6) Microscope instability. This is most commonly caused by contamination of the objective aperture, but dirt or lint can easily get on
specimen screens or the cap of the specimen holder and cause trouble.
Furthermore, dirt can be carried into the column with the specimen
holder and dropped down on the lip of the objective pole-piece. Of
course there may also be circuit instability. Difficulties of this sort can be
avoided easily if a practice is made of checking stability daily.
REASONS FOR UNSATISFACTORY PICTURES
309
only when he has completed the finished prints. In assembling the following summary, some effort has been made to arrange these difficulties in
order of frequency.
(1) Unduly thick sections. The beginner certainly will be tempted to
take pictures of sections that are too thick to show good resolution (Fig.
36). This is easily corrected with experience when the operator learns
to equate properly the contrast values of the fluorescent screen with those
that will appear on the photographic plate.
(2) Unduly thick supporting films. Any supporting film reduces resolution and contrast so that the only good film is one which barely sustains
the specimen. The operator should be willing to sacrifice many potential
grid squares in order to achieve quality in the surviving ones. The beginner should be warned that a fairly thick section on a thick supporting film
may look deceptively good on the fluorescent screen. It will have most of
the contrast properties of a thin section on a thin film, but will, of course,
lack the resolution of the latter.
(3) Specimen drift during picture taking. This was formerly a very
important source of trouble. It need not be serious now if cross-linked
plastic embedments are employed, if carbon films or carbon stabilized
films are used in conjunction with fairly rapid shutter speeds, and if
severely torn specimens are avoided.
(4) Changes in focus during preparations for photography. The beginner is apt to feel that he needs an unduly bright beam for accurate
focusing. If the operator then reduces the intensity greatly, there may be
a change in the focal plane of the objective lens so that the image he
photographs will be, in fact, out of focus. There is less danger of this
now than formerly because pictures are taken much more rapidly than
originally, but every operator should carefully explore and master the
characteristics of his instrument, and learn what the tolerances may be.
(5) Objective lens astigmatism. This limits the possibilities of obtaining high resolution pictures. It can be guarded against by making a daily
practice of checking image quality (Chapter 8.12).
(6) Microscope instability. This is most commonly caused by contamination of the objective aperture, but dirt or lint can easily get on
specimen screens or the cap of the specimen holder and cause trouble.
Furthermore, dirt can be carried into the column with the specimen
holder and dropped down on the lip of the objective pole-piece. Of
course there may also be circuit instability. Difficulties of this sort can be
avoided easily if a practice is made of checking stability daily.