9.3.
FOCUS
303
ing" procedures, however, usually make it possible to so increase the
density of proteinaceous materials that differences are minimized. Then
it becomes possible to obtain high contrast pictures at great resolution
from very thin sections. One can be confident that he is seeing all that
there is to be seen, even if he is using one of the newer cross-linked
plastic embedding media.
The technique of ultrathin sectioning is such today that it is easy to
cut sections too thin to make satisfactory micrographs without "staining/'
even when methacrylate is used as the embedding medium [Fig. 35(b)].
They may so lack contrast that they can be imaged only faintly on a
photographic plate, even though photographic processes are exploited
to produce the maximum contrast compatible with fine grain. Thus we
have become very dependent upon "stains" which allow us to use very
thin sections, and achieve resolutions which we hardly dared dream of
before 1958 (see Chapter 7.1-9).
The microscopist will have to learn mostly by experience what images
that he sees on the fluorescent screen will be useful on the photographic
plate. The fluorescent screen does not have the contrast or resolution
of the photographic plate, and the beginner will be lured into trying
to take pictures of too thick sections (Fig. 36). He must learn to resist
these, and photograph only those which have low contrast on the fluorescent screen.
9.3.
Focus
The proper focusing of an electron image is surprisingly difficult for
the beginner. The novice is apt to produce fuzzy pictures for some considerable time, and he is likely to blame all sorts of things except his
ability to focus properly. The main difficulty is that the most vivid image
with most apparent contrast ordinarily is a little out of focus (below
FIG. 36. (a) A transverse section of a small arteriole. This section was too thick for
good resolution (compare with Fig. 35). Such a specimen produces a high contrast image
on the fluorescent screen of the microscope. The novice is tempted to record it
photographically, but the resolution is so bad that one gains little or nothing by
photographically enlarging it later, (b) Shows a portion of this section photographed
originally at high magnification. Details are poorly resolved because of section thickness.
The micrograph is quite worthless for serious work. Tissue embedded in methacrylate,
unstained.
FOCUS
303
ing" procedures, however, usually make it possible to so increase the
density of proteinaceous materials that differences are minimized. Then
it becomes possible to obtain high contrast pictures at great resolution
from very thin sections. One can be confident that he is seeing all that
there is to be seen, even if he is using one of the newer cross-linked
plastic embedding media.
The technique of ultrathin sectioning is such today that it is easy to
cut sections too thin to make satisfactory micrographs without "staining/'
even when methacrylate is used as the embedding medium [Fig. 35(b)].
They may so lack contrast that they can be imaged only faintly on a
photographic plate, even though photographic processes are exploited
to produce the maximum contrast compatible with fine grain. Thus we
have become very dependent upon "stains" which allow us to use very
thin sections, and achieve resolutions which we hardly dared dream of
before 1958 (see Chapter 7.1-9).
The microscopist will have to learn mostly by experience what images
that he sees on the fluorescent screen will be useful on the photographic
plate. The fluorescent screen does not have the contrast or resolution
of the photographic plate, and the beginner will be lured into trying
to take pictures of too thick sections (Fig. 36). He must learn to resist
these, and photograph only those which have low contrast on the fluorescent screen.
9.3.
Focus
The proper focusing of an electron image is surprisingly difficult for
the beginner. The novice is apt to produce fuzzy pictures for some considerable time, and he is likely to blame all sorts of things except his
ability to focus properly. The main difficulty is that the most vivid image
with most apparent contrast ordinarily is a little out of focus (below
FIG. 36. (a) A transverse section of a small arteriole. This section was too thick for
good resolution (compare with Fig. 35). Such a specimen produces a high contrast image
on the fluorescent screen of the microscope. The novice is tempted to record it
photographically, but the resolution is so bad that one gains little or nothing by
photographically enlarging it later, (b) Shows a portion of this section photographed
originally at high magnification. Details are poorly resolved because of section thickness.
The micrograph is quite worthless for serious work. Tissue embedded in methacrylate,
unstained.