5.24.
ESTIMATION OF SECTION THICKNESS
185
can be arranged to reflect light from the meniscus of the fluid in the
trough. As the meniscus is changed it will be necessary to move the light
to maintain the proper reflection. If the knife edge has a peculiar shape,
perhaps a steep slope or a high spur on one side, the meniscus may have
such a complex geometry that viewing the sections in reflected light
becomes difficult or impossible. For this reason, knives with horizontal
edges and small spurs are desirable.
The best light source for viewing sections by reflected light is a
fluorescent
lamp. Since the meniscus is not usually flat, a broad source of
light helps. One also wishes to avoid the thermal effects of an incandescent
light source close to the block and knife. The new miniature
fluorescent
lights on a movable arm are probably best. The entire fixture can be
fitted in between the binocular microscope and the housing of the microtome, and casts a diffuse light over the whole trough area [Fig. 20(d)].
There is no simple answer as to what is desirable section thickness.
For survey work or histological work when relatively low magnifications
and resolutions are to be used, one wants much thicker sections than for
high magnification, high resolution work. The very thin section does
not have adequate contrast for survey work [Fig. 35(b)], as discussed in
detail in Chapter 9.2. Therefore, in most situations, one is happy to have
a series of sections showing a range of thickness. We have known for
some years that gray, silver, and pale gold sections spanned this useful
range. Deep gold colors are usually too thick to serve any useful purpose,
and purple sections certainly are.
Commonly, when one is cutting as large a block as he can get away
with, and when the Porter-Blum microtome is set to advance two units
with each cycle, there is considerable variation in section thickness, so that
one has gray, silver, and light gold sections produced together. When the
microtome is set at one unit of advance, all of the sections may be gray
and pale silver. One often can pick up groups of silver sections on one
screen, pale gold ones on another, and a series that are mostly gray on yet
another. If one really wants to obtain sections of great uniformity, for
instance for serial section work, the block face must be very small and
all other conditions of sectioning be favorable. It is then that the newer
automatic microtomes prove particularly valuable for their motor drives
can produce more standardized sections than possibly can be produced
by hand cycling a microtome.
The beginner probably should concentrate on pale gold sections for
there is no point in his trying to do high resolution work until he has
ESTIMATION OF SECTION THICKNESS
185
can be arranged to reflect light from the meniscus of the fluid in the
trough. As the meniscus is changed it will be necessary to move the light
to maintain the proper reflection. If the knife edge has a peculiar shape,
perhaps a steep slope or a high spur on one side, the meniscus may have
such a complex geometry that viewing the sections in reflected light
becomes difficult or impossible. For this reason, knives with horizontal
edges and small spurs are desirable.
The best light source for viewing sections by reflected light is a
fluorescent
lamp. Since the meniscus is not usually flat, a broad source of
light helps. One also wishes to avoid the thermal effects of an incandescent
light source close to the block and knife. The new miniature
fluorescent
lights on a movable arm are probably best. The entire fixture can be
fitted in between the binocular microscope and the housing of the microtome, and casts a diffuse light over the whole trough area [Fig. 20(d)].
There is no simple answer as to what is desirable section thickness.
For survey work or histological work when relatively low magnifications
and resolutions are to be used, one wants much thicker sections than for
high magnification, high resolution work. The very thin section does
not have adequate contrast for survey work [Fig. 35(b)], as discussed in
detail in Chapter 9.2. Therefore, in most situations, one is happy to have
a series of sections showing a range of thickness. We have known for
some years that gray, silver, and pale gold sections spanned this useful
range. Deep gold colors are usually too thick to serve any useful purpose,
and purple sections certainly are.
Commonly, when one is cutting as large a block as he can get away
with, and when the Porter-Blum microtome is set to advance two units
with each cycle, there is considerable variation in section thickness, so that
one has gray, silver, and light gold sections produced together. When the
microtome is set at one unit of advance, all of the sections may be gray
and pale silver. One often can pick up groups of silver sections on one
screen, pale gold ones on another, and a series that are mostly gray on yet
another. If one really wants to obtain sections of great uniformity, for
instance for serial section work, the block face must be very small and
all other conditions of sectioning be favorable. It is then that the newer
automatic microtomes prove particularly valuable for their motor drives
can produce more standardized sections than possibly can be produced
by hand cycling a microtome.
The beginner probably should concentrate on pale gold sections for
there is no point in his trying to do high resolution work until he has