140
5.
SECTIONING
forward in a horizontal plane at the moment of sectioning. It seems to
this author that such a motion is the steadiest and most controllable of
the whole cycle of the hand, although other workers might find that
pulling the handle toward them at the bottom of the turn might be an
equally good or preferred position for cutting sections. In any case, at the
critical moment of sectioning, the hand should be moving in a horizontal
plane rather than a vertical or oblique one.
It also seems desirable to put some sort of a drag on the microtome
which otherwise turns so freely that it is hard to control the speed very
precisely. In our own laboratory we originally mounted the flywheel in a
lathe and cut a V-shaped groove in its rim. A piece of heavy but flexible
cord, or leather belting, which was greased and fastened to the table at
one end, was then dangled over this to provide the drag. The amount of
the drag depended upon the length of the dangling cord or upon weights
which might be attached. Just enough drag was established so that one
had something to push against in cycling the instrument. This modification is partly visible in the lower right of Fig. 18. Subsequently, we
modified our Porter-Blum microtomes with a gear train as described
below [Fig. 20(d)], which provided enough drag in itself.
When the LKB-Produkter Ab. "Ultrotome" first was marketed, it
quickly achieved an excellent reputation. It soon became evident that
sectioning of the newer plastic embedments was best on this automatic
machine when it was used at its slowest cutting speed, 2 mm/sec. I then
made some simple approximate kymograph measurements of my own
cutting speeds using a standard Porter-Blum microtome. I found I could
not cut steadily and reliably at speeds appreciably less than 5 mm/sec.
We then devised the gear train for the Porter-Blum microtome illustrated
in Fig. 20(d), so that the crank had to be turned two (or four) full circles
to complete one cycle of the microtome. The reduced cutting speeds that
then became possible immediately resulted in better and more consistent
sectioning on the part of everyone associated with the laboratory. A cutting
speed of about 2 mm/sec is indeed about optimum for most sectioning as
we have since confirmed with the new Sorvall MT-2 microtome with which
the cutting speed can be controlled in a continuously variable manner
in this range. With "difficult" blocks one easily can demonstrate that
speed is a critical factor, and sectioning deteriorates when the speed is
either too rapid or too slow. Empirically, it would seem difficult with
hand operation of the standard Porter-Blum microtome to slow the
5.
SECTIONING
forward in a horizontal plane at the moment of sectioning. It seems to
this author that such a motion is the steadiest and most controllable of
the whole cycle of the hand, although other workers might find that
pulling the handle toward them at the bottom of the turn might be an
equally good or preferred position for cutting sections. In any case, at the
critical moment of sectioning, the hand should be moving in a horizontal
plane rather than a vertical or oblique one.
It also seems desirable to put some sort of a drag on the microtome
which otherwise turns so freely that it is hard to control the speed very
precisely. In our own laboratory we originally mounted the flywheel in a
lathe and cut a V-shaped groove in its rim. A piece of heavy but flexible
cord, or leather belting, which was greased and fastened to the table at
one end, was then dangled over this to provide the drag. The amount of
the drag depended upon the length of the dangling cord or upon weights
which might be attached. Just enough drag was established so that one
had something to push against in cycling the instrument. This modification is partly visible in the lower right of Fig. 18. Subsequently, we
modified our Porter-Blum microtomes with a gear train as described
below [Fig. 20(d)], which provided enough drag in itself.
When the LKB-Produkter Ab. "Ultrotome" first was marketed, it
quickly achieved an excellent reputation. It soon became evident that
sectioning of the newer plastic embedments was best on this automatic
machine when it was used at its slowest cutting speed, 2 mm/sec. I then
made some simple approximate kymograph measurements of my own
cutting speeds using a standard Porter-Blum microtome. I found I could
not cut steadily and reliably at speeds appreciably less than 5 mm/sec.
We then devised the gear train for the Porter-Blum microtome illustrated
in Fig. 20(d), so that the crank had to be turned two (or four) full circles
to complete one cycle of the microtome. The reduced cutting speeds that
then became possible immediately resulted in better and more consistent
sectioning on the part of everyone associated with the laboratory. A cutting
speed of about 2 mm/sec is indeed about optimum for most sectioning as
we have since confirmed with the new Sorvall MT-2 microtome with which
the cutting speed can be controlled in a continuously variable manner
in this range. With "difficult" blocks one easily can demonstrate that
speed is a critical factor, and sectioning deteriorates when the speed is
either too rapid or too slow. Empirically, it would seem difficult with
hand operation of the standard Porter-Blum microtome to slow the