206
6. SECTION MOUNTING
convenience recommends it. When high resolution work is contemplated,
a control is a wise precaution.
Sections supported by carbon evaporated directly upon them may, at
first, create a poor impression, for every little hole or tear in the original
section remains a hole which is glaringly conspicuous. Even under the
most favorable of circumstances, there are ordinarily at least a few small
holes in every grid square. But it should not be difficult to find very nicely
exhibited low power fields of epithelial tissues. If difficult material has
been sectioned, however, particularly if there was much collagen in the
specimen, there will be great numbers of large and small holes which will
make most low power fields look rather messy.
When a portion of section is not supported on all four sides by grid
wires, the evaporated carbon film usually does not offer sufficient support
and the section is lost in that square. Thus, if there are many scratches
in the sections, most potential squares may be lost and the yield of good
areas be small. For this reason the technique is mainly useful with material that has been well sectioned.
6.7. Carbon Stabilization of Plastic Films
Extrathin Parlodion or Formvar films can be stabilized by evaporating
a thin coat of carbon upon their surfaces. This gives them strength and
prevents the drift or creep that pure plastic films are apt to show. The
method is so simple and reliable that it can be recommended as the
method of choice for the beginner, and it is apt to remain the best routine way of preparing supporting films.
Parlodion or Formvar films are prepared as thin as will stand up on
the supporting screens. To do this, the stock solution of Parlodion or
Formvar is diluted as much as one can get away with. Then coated screens
are prepared as previously described. Finally, carbon is evaporated on the
screens until the film is just visible on the porcelain indicator. Once
screens are prepared this way, they can be stored in covered dishes until
one is ready to use them. Thus, a considerable number of screens can be
made at one time.
The only important disadvantage of this method is that the combined
film is slightly thicker than is actually needed. Thus, one does not have
quite the contrast and potential resolution that can be obtained using
6. SECTION MOUNTING
convenience recommends it. When high resolution work is contemplated,
a control is a wise precaution.
Sections supported by carbon evaporated directly upon them may, at
first, create a poor impression, for every little hole or tear in the original
section remains a hole which is glaringly conspicuous. Even under the
most favorable of circumstances, there are ordinarily at least a few small
holes in every grid square. But it should not be difficult to find very nicely
exhibited low power fields of epithelial tissues. If difficult material has
been sectioned, however, particularly if there was much collagen in the
specimen, there will be great numbers of large and small holes which will
make most low power fields look rather messy.
When a portion of section is not supported on all four sides by grid
wires, the evaporated carbon film usually does not offer sufficient support
and the section is lost in that square. Thus, if there are many scratches
in the sections, most potential squares may be lost and the yield of good
areas be small. For this reason the technique is mainly useful with material that has been well sectioned.
6.7. Carbon Stabilization of Plastic Films
Extrathin Parlodion or Formvar films can be stabilized by evaporating
a thin coat of carbon upon their surfaces. This gives them strength and
prevents the drift or creep that pure plastic films are apt to show. The
method is so simple and reliable that it can be recommended as the
method of choice for the beginner, and it is apt to remain the best routine way of preparing supporting films.
Parlodion or Formvar films are prepared as thin as will stand up on
the supporting screens. To do this, the stock solution of Parlodion or
Formvar is diluted as much as one can get away with. Then coated screens
are prepared as previously described. Finally, carbon is evaporated on the
screens until the film is just visible on the porcelain indicator. Once
screens are prepared this way, they can be stored in covered dishes until
one is ready to use them. Thus, a considerable number of screens can be
made at one time.
The only important disadvantage of this method is that the combined
film is slightly thicker than is actually needed. Thus, one does not have
quite the contrast and potential resolution that can be obtained using