6.4.
PREPARING FILM NETS
199
more difficult to remove. Again the beginner should be advised not to
clean the microscope slide too thoroughly, for this favors undesirably
strong adhesion between glass and plastic.
6.4. Preparing Film Nets
Because supporting films inevitably reduce contrast and limit resolution, efforts have been made to avoid using them. Two approaches obviously are possible. One is to use finer and finer mesh supporting grids
until the holes are so small that naked specimens stretched across them
survive. Modern technology in making grids allows this to be done, and
grids of almost any mesh size can be ordered.
It is also possible to make films of Parlodion or Formvar which are in
the form of fine nets rather than being continuous sheets. These nets,
when stretched over conventional screens, and particularly after they
have been stabilized with carbon (see below), provide a simple solution.
Such nets, as described by Sjostrand (1957), are easy to prepare by condensing moisture droplets on the surface of the film just before it
hardens. A glass slide is dipped in a 2% solution of Formvar in ethylene
dichloride. It is then drained for about 20 minutes in an atmosphere
saturated with this solvent. Finally, one can breathe on the surface so
that it "frosts/' or it is easy to blow through a flask of hot water and
create an atmosphere supersaturated with water which will condense on
the glass surface. The nets so formed are floated off the slide, and stabilized with carbon (see below). The average size of the holes so formed is
controllable within limits by varying the timing and degree of condensation. More simply the glass slide may be dried at once while observing
its surface with reflected light. As soon as interference colors begin to
appear, it is time to begin lightly "frosting" the film by breathing on
it. As the drying proceeds across the slide, this is continued. The condensed moisture droplets create the holes. Very coarse nets can be prepared if grids previously coated with this sort of Formvar film are heavily
bombarded in the electron beam of the microscope before mounting a
specimen. The holes can be observed growing during the bombardment,
and their size can be controlled at will. Finally the net can be stabilized
by evaporating carbon upon it. Naturally, since one is interested in the
holes, the film can be thicker and stronger than otherwise would be
permissible.
PREPARING FILM NETS
199
more difficult to remove. Again the beginner should be advised not to
clean the microscope slide too thoroughly, for this favors undesirably
strong adhesion between glass and plastic.
6.4. Preparing Film Nets
Because supporting films inevitably reduce contrast and limit resolution, efforts have been made to avoid using them. Two approaches obviously are possible. One is to use finer and finer mesh supporting grids
until the holes are so small that naked specimens stretched across them
survive. Modern technology in making grids allows this to be done, and
grids of almost any mesh size can be ordered.
It is also possible to make films of Parlodion or Formvar which are in
the form of fine nets rather than being continuous sheets. These nets,
when stretched over conventional screens, and particularly after they
have been stabilized with carbon (see below), provide a simple solution.
Such nets, as described by Sjostrand (1957), are easy to prepare by condensing moisture droplets on the surface of the film just before it
hardens. A glass slide is dipped in a 2% solution of Formvar in ethylene
dichloride. It is then drained for about 20 minutes in an atmosphere
saturated with this solvent. Finally, one can breathe on the surface so
that it "frosts/' or it is easy to blow through a flask of hot water and
create an atmosphere supersaturated with water which will condense on
the glass surface. The nets so formed are floated off the slide, and stabilized with carbon (see below). The average size of the holes so formed is
controllable within limits by varying the timing and degree of condensation. More simply the glass slide may be dried at once while observing
its surface with reflected light. As soon as interference colors begin to
appear, it is time to begin lightly "frosting" the film by breathing on
it. As the drying proceeds across the slide, this is continued. The condensed moisture droplets create the holes. Very coarse nets can be prepared if grids previously coated with this sort of Formvar film are heavily
bombarded in the electron beam of the microscope before mounting a
specimen. The holes can be observed growing during the bombardment,
and their size can be controlled at will. Finally the net can be stabilized
by evaporating carbon upon it. Naturally, since one is interested in the
holes, the film can be thicker and stronger than otherwise would be
permissible.