5.17.
TROUGH CONSTRUCTION
167
Naturally adhesive tape cannot be made to fit snugly across the
irregular back of the knife. Therefore, this region of the heel of the
trough is filled with wax to make a seal. Filtered beeswax is very effective.
This wax is tacky and adheres to both the glass and the adhesive quite
securely. It has a low melting point, and a pot of it can be kept melted
easily in any paraffin oven. It does not contaminate the acetone solutions
used in the trough. A small camel's hair brush is kept in the pot, and the
hot wax built up both inside and outside of the trough in the region of
the heel to complete the seal.
The adhesives used on adhesive tapes are a potential source of danger
as far as contaminating the knife edge is concerned. The adhesive should
be suspect if the glass near the knife edge becomes hydrophobic as it is
used. This can be avoided if the interior of the trough is painted with
hot beeswax. Waterproof adhesive tapes can be found, however, which do
not contaminate, and can be used without this precaution. Manufacturing companies have been "improving" and changing their adhesives
in recent years so that a tape which is satisfactory at one time may not be
when reordered.
Although the above method of making troughs seems quite reliable
and easy, many laboratories prefer to use metal troughs made from sheet
copper or aluminum. Some microtome manufacturing companies sell
metal troughs made of bronze or stainless steel. These are cut and bent
appropriately [Fig. 19(a)], and are fastened to the knife with molten
FIG. 18. The region of the knife and block holder of a Porter-Blum microtome.
A glass knife with a trough made of adhesive tape and sealed with wax is in its holder.
Note that the front face of the knife is nearly, but not quite, vertical. Also, the knife is
snugly down in the holder for maximum support and to prevent its edge from being
at too high a level. The block must pass the knife edge during the middle portion of
its downstroke.
This figure indicates how specimens may be removed from the trough using a glass
spatula as described in Chapter 5.26. If the glass previously is wet with the same acetone
solution as in the trough, surface tension will pull sections up on the glass as acetone
rapidly evaporates from the thin film on the spatula.
This microtome has been modified slightly in that a V-groove was cut in the fly
wheel, and a leather belt fastened to the table at one end dangles over it. This serves
as a brake as described in Chapter 5.3. A massive chuck also has been provided which
adds inertia to the system. The microtome is being used with a binocular dissecting
microscope mounted on an extension arm and having a very long focal length objective,
visible in the upper left of the
figure.
TROUGH CONSTRUCTION
167
Naturally adhesive tape cannot be made to fit snugly across the
irregular back of the knife. Therefore, this region of the heel of the
trough is filled with wax to make a seal. Filtered beeswax is very effective.
This wax is tacky and adheres to both the glass and the adhesive quite
securely. It has a low melting point, and a pot of it can be kept melted
easily in any paraffin oven. It does not contaminate the acetone solutions
used in the trough. A small camel's hair brush is kept in the pot, and the
hot wax built up both inside and outside of the trough in the region of
the heel to complete the seal.
The adhesives used on adhesive tapes are a potential source of danger
as far as contaminating the knife edge is concerned. The adhesive should
be suspect if the glass near the knife edge becomes hydrophobic as it is
used. This can be avoided if the interior of the trough is painted with
hot beeswax. Waterproof adhesive tapes can be found, however, which do
not contaminate, and can be used without this precaution. Manufacturing companies have been "improving" and changing their adhesives
in recent years so that a tape which is satisfactory at one time may not be
when reordered.
Although the above method of making troughs seems quite reliable
and easy, many laboratories prefer to use metal troughs made from sheet
copper or aluminum. Some microtome manufacturing companies sell
metal troughs made of bronze or stainless steel. These are cut and bent
appropriately [Fig. 19(a)], and are fastened to the knife with molten
FIG. 18. The region of the knife and block holder of a Porter-Blum microtome.
A glass knife with a trough made of adhesive tape and sealed with wax is in its holder.
Note that the front face of the knife is nearly, but not quite, vertical. Also, the knife is
snugly down in the holder for maximum support and to prevent its edge from being
at too high a level. The block must pass the knife edge during the middle portion of
its downstroke.
This figure indicates how specimens may be removed from the trough using a glass
spatula as described in Chapter 5.26. If the glass previously is wet with the same acetone
solution as in the trough, surface tension will pull sections up on the glass as acetone
rapidly evaporates from the thin film on the spatula.
This microtome has been modified slightly in that a V-groove was cut in the fly
wheel, and a leather belt fastened to the table at one end dangles over it. This serves
as a brake as described in Chapter 5.3. A massive chuck also has been provided which
adds inertia to the system. The microtome is being used with a binocular dissecting
microscope mounted on an extension arm and having a very long focal length objective,
visible in the upper left of the
figure.