8. Microscopy
8.1. Introductory Remarks
The biological investigator working with ultrathin sections is involved in a never ceasing struggle to improve contrast. For him, contrast
is an intrinsic difficulty since his material inevitably consists of substances
made up of light atoms having approximately the same density as the
embedding media and supporting films. It would be fair to say that any
given specimen has a certain potential contrast which we can only
partially realize by the most skillful microscopy and modern photography.
To a large extent contrast depends inversely upon the amount of
"background fog" that the photographic plate records. Fogging results
from scattered electrons, one way or another reaching the photographic
emulsion. These electrons travel at varying speeds, and therefore are not
focusable. They do not particularly traverse the refined optical pathway
that we are interested in, but many spray the plate randomly after having
bounced off the sidewalls of the microscope column. It is for this reason
that very important steps can be taken to minimize the number of
scattered electrons which finally reach the viewing screen and photographic emulsion. If apertures of suitably small size are placed at critical
levels along the microscope column, the useful beam can go through
these tiny holes while scattered electrons, even though they may be
nearby, will hit the aperture plate and effectively be removed.
The original post-war microscopes were not apertured at all in the
modern sense. Such microscopes are perfectly useful today if used solely
with specimens of great intrinsic contrast. These include particulate
materials and surface replicas which have been shadowed with heavy
metals. The first type of preparation is commonly used by bacteriologists
and virologists, the second type, by metallurgists. Unapertured microscopes are also used for the direct observation of dense particulate matter
as in dealing with pigments, smokes, etc. Investigators in these fields have
a much easier time with their microscopy than those of us who are
interested in tissue work with ultrathin sections.
The widespread use of objective apertures pretty much coincided with
the development of ultrathin sectioning (Fig. 30). It was the RCA EMU265
8.1. Introductory Remarks
The biological investigator working with ultrathin sections is involved in a never ceasing struggle to improve contrast. For him, contrast
is an intrinsic difficulty since his material inevitably consists of substances
made up of light atoms having approximately the same density as the
embedding media and supporting films. It would be fair to say that any
given specimen has a certain potential contrast which we can only
partially realize by the most skillful microscopy and modern photography.
To a large extent contrast depends inversely upon the amount of
"background fog" that the photographic plate records. Fogging results
from scattered electrons, one way or another reaching the photographic
emulsion. These electrons travel at varying speeds, and therefore are not
focusable. They do not particularly traverse the refined optical pathway
that we are interested in, but many spray the plate randomly after having
bounced off the sidewalls of the microscope column. It is for this reason
that very important steps can be taken to minimize the number of
scattered electrons which finally reach the viewing screen and photographic emulsion. If apertures of suitably small size are placed at critical
levels along the microscope column, the useful beam can go through
these tiny holes while scattered electrons, even though they may be
nearby, will hit the aperture plate and effectively be removed.
The original post-war microscopes were not apertured at all in the
modern sense. Such microscopes are perfectly useful today if used solely
with specimens of great intrinsic contrast. These include particulate
materials and surface replicas which have been shadowed with heavy
metals. The first type of preparation is commonly used by bacteriologists
and virologists, the second type, by metallurgists. Unapertured microscopes are also used for the direct observation of dense particulate matter
as in dealing with pigments, smokes, etc. Investigators in these fields have
a much easier time with their microscopy than those of us who are
interested in tissue work with ultrathin sections.
The widespread use of objective apertures pretty much coincided with
the development of ultrathin sectioning (Fig. 30). It was the RCA EMU265