178
5.
SECTIONING
depends upon three other factors: (1) the proper (slower than you think)
cutting speed, (2) a steady cycling rhythm, (3) a light touch if the cycling
is being done by hand.
The cutting speed is quite important. Optimum speed varies somewhat
depending upon the plastic being cut and its hardness. Relatively soft
methacrylate will probably section best with a speed of about 5 mm/sec.
Hard Araldite or Vestopal W blocks normally will cut best at about one
half that speed, approximately 2 mm/sec. Some automatic microtomes do
not section this slowly, and this is a serious limitation. As indicated
earlier (Chapter 5.3.), it is difficult (but not impossible) to cycle the
original Porter-Blum instrument by hand this slowly, but it can be
modified so that this can be accomplished easily.
The cutting speed of the automatic microtomes is, of course, controlled mechanically. In hand operation, however, one learns to judge
this by trying to follow the movement of the block with one's eyes while
looking through the binocular viewing microscope. If it is not possible to
keep one's eyes on the block as it sweeps across the microscope field, the
pass is almost certainly too fast. When the block is going slowly enough
for effective sectioning, it should be easy to keep one's eyes on it. There
is a strong unconscious tendency to speed up the cutting when apparently
sections are being cut successfully, but excessive cutting speeds lead to
the fine-order chatter discussed below. Thus, even though one may get
sections with fast cutting, these are apt to be worthless.
In hand operation a steady rhythm is essential for successful ultrathin
sectioning. The microtome can, and should, be cycled fairly rapidly
except during the actual cutting phase, for a short period favors reproducibility, no doubt partly by minimizing thermal fluctuations. A
comfortable rhythm that can be maintained is perfected only with
practice. Until this skill is developed, sections will be more variable than
otherwise might be the case.
In general, then, the cycling motion is divided into two phases. The
first is the fairly rapid phase which begins after a section has been cut and
the block has disappeared from view behind the knife, and which ends perhaps with a momentary pause as the block is poised above the knife, ready
to come down across the edge. The second phase is the slow vertical
descent of the block across the knife edge. It is, of course, important that
this last be as steady and as controlled a motion as is possible. This author
thinks that the operator's hand and arm should be either pushing or
pulling in the horizontal plane during the second phase of the cycle, and
5.
SECTIONING
depends upon three other factors: (1) the proper (slower than you think)
cutting speed, (2) a steady cycling rhythm, (3) a light touch if the cycling
is being done by hand.
The cutting speed is quite important. Optimum speed varies somewhat
depending upon the plastic being cut and its hardness. Relatively soft
methacrylate will probably section best with a speed of about 5 mm/sec.
Hard Araldite or Vestopal W blocks normally will cut best at about one
half that speed, approximately 2 mm/sec. Some automatic microtomes do
not section this slowly, and this is a serious limitation. As indicated
earlier (Chapter 5.3.), it is difficult (but not impossible) to cycle the
original Porter-Blum instrument by hand this slowly, but it can be
modified so that this can be accomplished easily.
The cutting speed of the automatic microtomes is, of course, controlled mechanically. In hand operation, however, one learns to judge
this by trying to follow the movement of the block with one's eyes while
looking through the binocular viewing microscope. If it is not possible to
keep one's eyes on the block as it sweeps across the microscope field, the
pass is almost certainly too fast. When the block is going slowly enough
for effective sectioning, it should be easy to keep one's eyes on it. There
is a strong unconscious tendency to speed up the cutting when apparently
sections are being cut successfully, but excessive cutting speeds lead to
the fine-order chatter discussed below. Thus, even though one may get
sections with fast cutting, these are apt to be worthless.
In hand operation a steady rhythm is essential for successful ultrathin
sectioning. The microtome can, and should, be cycled fairly rapidly
except during the actual cutting phase, for a short period favors reproducibility, no doubt partly by minimizing thermal fluctuations. A
comfortable rhythm that can be maintained is perfected only with
practice. Until this skill is developed, sections will be more variable than
otherwise might be the case.
In general, then, the cycling motion is divided into two phases. The
first is the fairly rapid phase which begins after a section has been cut and
the block has disappeared from view behind the knife, and which ends perhaps with a momentary pause as the block is poised above the knife, ready
to come down across the edge. The second phase is the slow vertical
descent of the block across the knife edge. It is, of course, important that
this last be as steady and as controlled a motion as is possible. This author
thinks that the operator's hand and arm should be either pushing or
pulling in the horizontal plane during the second phase of the cycle, and