5.22.
SECTIONING
179
therefore recommended in Chapter 5.3 that the microtome handle be
rotated on its shaft to establish this correspondence.
The importance of maintaining a steady rhythm is at once apparent
after there has been an interruption for one reason or another. Thermal
changes will have occurred during the interruption and the first cycle may
produce a thick section, or conversely, nothing, and the knife may have
to be moved into the block face with delicate adjustments. The net effect
of this will also be to produce a thick section. These thick sections, no
doubt, set up undue compression effects within the block, so that as one
begins a sectioning rhythm the first sections are apt to be of rather
varying thickness, and there may be considerable skipping. It is only after
perhaps
a dozen or more cycles that the microtome
and the block will
settle down so that reasonably
uniform sections can be cut. The experienced operator, thus, hates an interruption when he is cutting well. One
sort of interruption is inevitable when enough sections have been cut so
that the ribbon bumps into the trough edge, or into rafts of other sections
and starts showing signs of serious back pressures which prevent the
proper expansion of new sections. Then an interruption must be accepted
to remove the sections from the trough. More annoying is a telephone call
which had better be ignored while good sectioning continues. Still more
annoying is someone in the laboratory who wishes to talk and divert
attention. The operator will do well to do no more than grunt replies
without looking up.
A "light touch" is very important in ultramicrotomy. In thinking about
this one should bear in mind that the sections he hopes to obtain will be
at most only a few hundred Angstrom units thick. One naturally does
not want much thickness variation within a single section. The machinery
of the microtome cannot be allowed to deviate more than perhaps a
hundred Angstrom units during the cycling or the actual sectioning.
We are approaching the dimensions of modest sized molecules, and we
are faced with the problem of keeping within these tolerances in quite a
bulk of microtome. Even massive castings can be expected to bend this
much when moderate pressures are applied. Quite apart from theory, it is
empirical experience that the operator can distort the microtome enough
to ruin thin sectioning simply by keeping a tight grip on the microtome
handle, and cranking it around as he might be used to doing in conventional microtomy. A truly delicate touch is indicated, and only fingertips should touch the handle lightly during the cutting portion of the
cycle when turning a microtome by hand. Indeed, this author has
SECTIONING
179
therefore recommended in Chapter 5.3 that the microtome handle be
rotated on its shaft to establish this correspondence.
The importance of maintaining a steady rhythm is at once apparent
after there has been an interruption for one reason or another. Thermal
changes will have occurred during the interruption and the first cycle may
produce a thick section, or conversely, nothing, and the knife may have
to be moved into the block face with delicate adjustments. The net effect
of this will also be to produce a thick section. These thick sections, no
doubt, set up undue compression effects within the block, so that as one
begins a sectioning rhythm the first sections are apt to be of rather
varying thickness, and there may be considerable skipping. It is only after
perhaps
a dozen or more cycles that the microtome
and the block will
settle down so that reasonably
uniform sections can be cut. The experienced operator, thus, hates an interruption when he is cutting well. One
sort of interruption is inevitable when enough sections have been cut so
that the ribbon bumps into the trough edge, or into rafts of other sections
and starts showing signs of serious back pressures which prevent the
proper expansion of new sections. Then an interruption must be accepted
to remove the sections from the trough. More annoying is a telephone call
which had better be ignored while good sectioning continues. Still more
annoying is someone in the laboratory who wishes to talk and divert
attention. The operator will do well to do no more than grunt replies
without looking up.
A "light touch" is very important in ultramicrotomy. In thinking about
this one should bear in mind that the sections he hopes to obtain will be
at most only a few hundred Angstrom units thick. One naturally does
not want much thickness variation within a single section. The machinery
of the microtome cannot be allowed to deviate more than perhaps a
hundred Angstrom units during the cycling or the actual sectioning.
We are approaching the dimensions of modest sized molecules, and we
are faced with the problem of keeping within these tolerances in quite a
bulk of microtome. Even massive castings can be expected to bend this
much when moderate pressures are applied. Quite apart from theory, it is
empirical experience that the operator can distort the microtome enough
to ruin thin sectioning simply by keeping a tight grip on the microtome
handle, and cranking it around as he might be used to doing in conventional microtomy. A truly delicate touch is indicated, and only fingertips should touch the handle lightly during the cutting portion of the
cycle when turning a microtome by hand. Indeed, this author has