212
6.
SECTION MOUNTING
with carbon substrates, and with sections upon them, are inverted on
top of the floating film. They are then picked up from the water surface
as described earlier in this chapter, and are ready for examination after
drying. It is suggested that the thickness of the added methacrylate film
be adjusted so that after electron bombardment the interference colors of
the section are the same as they were to begin with. Thus, what originally
was a pale gold section might show a dark gold color after being mounted
on the substrate. The addition of the methacrylate film would then, perhaps, produce purple, blue, or green colors. However, after the section
is bombarded in the electron microscope and some of the methacrylate
decomposed, the section should return to the dark gold color.
6.11. Serial Sections
It is not difficult to cut reasonably uniform serial sections. It should
be realized, however, that the full series of sections to be examined almost
has to be put on a single grid, for hazards multiply in trying to continue
a series on several grids. Thus, the block must be trimmed so that the
section width is very narrow and the number of sections needed will be
included in a ribbon approximately 1/16 in. long.
One cannot use the ordinary supporting grids for serial section work.
Originally grids with long slots in them rather than square holes were
used. This necessitated a rather exact orientation of the ribbon with the
slot, and inevitably accomplishing this had its hazards. Furthermore, the
physical forces working on a supporting film of a narrow rectangular
shape are different in the two axes, and such films are more fragile than
they would be if symmetrically supported. Thus it now seems better to use
a supporting grid with a single spherical or nearly spherical hole in it as
described by Dowell (1959). The larger the hole, of course, the stronger
the supporting film must be. All things being equal, this means that the
film has to be thicker than it otherwise would be, and one must pay for
this with the loss of some contrast and resolution. However, carbon-coated
Formvar or Parlodion films (Chapter 6.7) serve well, our modern staining
techniques provide adequate contrast, and microscopes with 80 or 100 kv
power supplies prove to be adequate for almost all such purposes. With
the modern automatic microtomes, a long ribbon of serial sections, much
longer than a grid is wide, can be obtained without difficulty. Then, if
6.
SECTION MOUNTING
with carbon substrates, and with sections upon them, are inverted on
top of the floating film. They are then picked up from the water surface
as described earlier in this chapter, and are ready for examination after
drying. It is suggested that the thickness of the added methacrylate film
be adjusted so that after electron bombardment the interference colors of
the section are the same as they were to begin with. Thus, what originally
was a pale gold section might show a dark gold color after being mounted
on the substrate. The addition of the methacrylate film would then, perhaps, produce purple, blue, or green colors. However, after the section
is bombarded in the electron microscope and some of the methacrylate
decomposed, the section should return to the dark gold color.
6.11. Serial Sections
It is not difficult to cut reasonably uniform serial sections. It should
be realized, however, that the full series of sections to be examined almost
has to be put on a single grid, for hazards multiply in trying to continue
a series on several grids. Thus, the block must be trimmed so that the
section width is very narrow and the number of sections needed will be
included in a ribbon approximately 1/16 in. long.
One cannot use the ordinary supporting grids for serial section work.
Originally grids with long slots in them rather than square holes were
used. This necessitated a rather exact orientation of the ribbon with the
slot, and inevitably accomplishing this had its hazards. Furthermore, the
physical forces working on a supporting film of a narrow rectangular
shape are different in the two axes, and such films are more fragile than
they would be if symmetrically supported. Thus it now seems better to use
a supporting grid with a single spherical or nearly spherical hole in it as
described by Dowell (1959). The larger the hole, of course, the stronger
the supporting film must be. All things being equal, this means that the
film has to be thicker than it otherwise would be, and one must pay for
this with the loss of some contrast and resolution. However, carbon-coated
Formvar or Parlodion films (Chapter 6.7) serve well, our modern staining
techniques provide adequate contrast, and microscopes with 80 or 100 kv
power supplies prove to be adequate for almost all such purposes. With
the modern automatic microtomes, a long ribbon of serial sections, much
longer than a grid is wide, can be obtained without difficulty. Then, if