9.2.
SECTION THICKNESS VERSUS MAGNIFICATION
301
Fic. 35. Closely adjacent sections of the same arteriole as that shown in Fig. 36.
(a) is of a section having very satisfactory thickness for a low magnification micrograph.
Cytological detail is sharp and has good contrast. Such a negative can be enlarged until
photographic grain interferes with image definition, (b) is of a section so thin that there
was inadequate contrast for a low magnification picture. The negative was excessively
"flat." In spite of adequate exposure, and even though this print was made on as hard
a paper as possible, the reproduction is unsatisfactory. Such a very thin section might
be useful for high magnification work, although one easily can have sections so thin
that high magnification micrographs also lack adequate contrast. Tissue fixed in osmium
tetroxide, embedded in methacrylate, and unstained. If a stain had been used the
contrast of the very thin section (b) might have been augmented to the point that
micrography would have been satisfactory. Surely this would have been the case for
high resolution microscopy.
It is at the root of many criticisms that laboratories of electron microscopy are not sufficiently productive.
9.2. Section Thickness versus Magnification
To take low magnification pictures, one needs a different sort of
section than when striving for very high magnifications. At low magnification, one wishes a large range of contrast in the picture. The "gray scale"
should be so long that subtle variations in density can be seen. Relatively
thick sections answer these requirements best, yet the sections must be
thin enough so that one does not sacrifice resolution appropriate to the
magnification [Fig. 35(a)].
At high magnification one usually is concerned with less complex
pictures where a long gray scale is not important. One is content seeing
objects in sharp contrast, black against white, almost as in an etching.
Of course, one also needs great resolution. These requirements are
satisfied by relatively thin sections.
In practice, then, no single section is apt to be optimal for both low
and high magnification work. The problem is complicated further since
some cells or organelles which are intrinsically very dense, or which have
been "stained," will have to be sectioned much more thinly than others
of low intrinsic density to achieve the same final photographic contrast.
This is well illustrated in considerating a specimen such as a transverse
section of a myelinated nerve fiber after osmium tetroxide fixation, in
which the myelin and axoplasm differ so greatly in density that both
can hardly be shown successfully on a single micrograph. Modern "stain-
SECTION THICKNESS VERSUS MAGNIFICATION
301
Fic. 35. Closely adjacent sections of the same arteriole as that shown in Fig. 36.
(a) is of a section having very satisfactory thickness for a low magnification micrograph.
Cytological detail is sharp and has good contrast. Such a negative can be enlarged until
photographic grain interferes with image definition, (b) is of a section so thin that there
was inadequate contrast for a low magnification picture. The negative was excessively
"flat." In spite of adequate exposure, and even though this print was made on as hard
a paper as possible, the reproduction is unsatisfactory. Such a very thin section might
be useful for high magnification work, although one easily can have sections so thin
that high magnification micrographs also lack adequate contrast. Tissue fixed in osmium
tetroxide, embedded in methacrylate, and unstained. If a stain had been used the
contrast of the very thin section (b) might have been augmented to the point that
micrography would have been satisfactory. Surely this would have been the case for
high resolution microscopy.
It is at the root of many criticisms that laboratories of electron microscopy are not sufficiently productive.
9.2. Section Thickness versus Magnification
To take low magnification pictures, one needs a different sort of
section than when striving for very high magnifications. At low magnification, one wishes a large range of contrast in the picture. The "gray scale"
should be so long that subtle variations in density can be seen. Relatively
thick sections answer these requirements best, yet the sections must be
thin enough so that one does not sacrifice resolution appropriate to the
magnification [Fig. 35(a)].
At high magnification one usually is concerned with less complex
pictures where a long gray scale is not important. One is content seeing
objects in sharp contrast, black against white, almost as in an etching.
Of course, one also needs great resolution. These requirements are
satisfied by relatively thin sections.
In practice, then, no single section is apt to be optimal for both low
and high magnification work. The problem is complicated further since
some cells or organelles which are intrinsically very dense, or which have
been "stained," will have to be sectioned much more thinly than others
of low intrinsic density to achieve the same final photographic contrast.
This is well illustrated in considerating a specimen such as a transverse
section of a myelinated nerve fiber after osmium tetroxide fixation, in
which the myelin and axoplasm differ so greatly in density that both
can hardly be shown successfully on a single micrograph. Modern "stain-