5.3.
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139
affecting the section thickness as is the case if a thermal advance is
employed.
The disadvantages of the original Porter-Blum microtome include
most importantly the limited number of specimen advance increments
that are useful, and difficulties in cutting as slowly as one would like
(the latter problem can be corrected by a modification described in
Chapter 5.3.). It must be admitted also that operating the instrument to
produce good uniform sections is something of a delicate art (Chapter
5.21), which is not so much a factor in using automatic microtomes.
The advance mechanism of this microtome is in terms of 250 A increments. In practice one has to do almost all of one's sectioning at a nominal
500 A advance, for circumstances rarely allow one to section well and
long at 250 A, and 750 A sections tend to be too thick for good resolution.
This limitation is not too serious, however, for actual section thickness
can be expected to vary somewhat, particularly in a hand-driven microtome. Thus, sections of satisfactory thickness ordinarily can be found
to meet all but the most exacting demands.
It should be noted, too, that this microtome can be used with a thermal
expansion drive. The prototype model, in fact, lacked a mechanical drive.
Advance was made by moving a lamp bulb close to the arm which carries
the specimen. This writer also knows of instruments which have been
modified by wrapping the specimen arm with Nichrome wire, which then
was used with a potentiometer control as a heating unit to elongate the
specimen arm while sections were being cut.
Any microtome used for ultrathin sectioning must be used with a
binocular "dissecting" microscope. In this instance a microscope on a
heavy independent arm-stand seems best. An adjustable arm attachment
for mounting a microscope directly on the Porter-Blum microtome is
available, but I have not found this to be very satisfactory, mainly because
its adjustment is difficult.
5,3. Modification of the Porter-Blum Microtome
In this writer's opinion, the Porter-Blum microtome should be modified
in several ways to make its operation more effective.
An important modification simply involves turning the flywheel on its
shaft so that the handle approaches the top of its turn just about at the
same time as the block is cut. Thus, the operator's hand is pushing
PORTER-BLUM MICROTOME
139
affecting the section thickness as is the case if a thermal advance is
employed.
The disadvantages of the original Porter-Blum microtome include
most importantly the limited number of specimen advance increments
that are useful, and difficulties in cutting as slowly as one would like
(the latter problem can be corrected by a modification described in
Chapter 5.3.). It must be admitted also that operating the instrument to
produce good uniform sections is something of a delicate art (Chapter
5.21), which is not so much a factor in using automatic microtomes.
The advance mechanism of this microtome is in terms of 250 A increments. In practice one has to do almost all of one's sectioning at a nominal
500 A advance, for circumstances rarely allow one to section well and
long at 250 A, and 750 A sections tend to be too thick for good resolution.
This limitation is not too serious, however, for actual section thickness
can be expected to vary somewhat, particularly in a hand-driven microtome. Thus, sections of satisfactory thickness ordinarily can be found
to meet all but the most exacting demands.
It should be noted, too, that this microtome can be used with a thermal
expansion drive. The prototype model, in fact, lacked a mechanical drive.
Advance was made by moving a lamp bulb close to the arm which carries
the specimen. This writer also knows of instruments which have been
modified by wrapping the specimen arm with Nichrome wire, which then
was used with a potentiometer control as a heating unit to elongate the
specimen arm while sections were being cut.
Any microtome used for ultrathin sectioning must be used with a
binocular "dissecting" microscope. In this instance a microscope on a
heavy independent arm-stand seems best. An adjustable arm attachment
for mounting a microscope directly on the Porter-Blum microtome is
available, but I have not found this to be very satisfactory, mainly because
its adjustment is difficult.
5,3. Modification of the Porter-Blum Microtome
In this writer's opinion, the Porter-Blum microtome should be modified
in several ways to make its operation more effective.
An important modification simply involves turning the flywheel on its
shaft so that the handle approaches the top of its turn just about at the
same time as the block is cut. Thus, the operator's hand is pushing