9.4.
SPECIAL PHOTOGRAPHIC CONSIDERATIONS
307
"cold stage." These are really cold traps which prevent organic vapors
from reaching the specimen. Commercial designs have begun to appear
as accessories for certain microscopes. But their nuisance as well as their
cost make it unlikely that they will find favor for routine purposes.
Rapid photography becomes possible with knowledge. If one is
beginning a new investigation and needs time to get oriented, it is
best not even to attempt photography at first but use grids solely for
visual observation. There comes a time when the investigator has gained
confidence and has a good idea of what he wants to record on the film.
Then, as he comes to each new grid square, he should decide without
delay if anything is worthwhile recording. This decision must take
into account section thickness and contrast which will determine whether
high or low power pictures are feasible. No unnecessary time should be
wasted in focusing and taking the picture. However, since it is useless to
take an out-of-focus picture, all the time that is really needed to find
a suitable focus object, and to be sure of accurate focus should be used.
Just after focusing has been completed, one should pause and be certain
that there is no specimen drift which would ruin a picture. This is
particularly important when unstabilized plastic supporting films are
being used at relatively high magnification. They are then being subjected to an intense electron bombardment which produces strains as
differential decomposition occurs. A moving specimen ordinarily should
be abandoned, although very occasionally it will become stable if the
beam current is reduced. It is particularly dangerous with unstabilized
plastic supporting films to try to take pictures of squares which have the
film split across the square, or which have pulled away from one supporting wire. Such films almost invariably show movement. The great advantage of stabilizing plastic supporting films with evaporated carbon
is that the specimen may remain motionless even when quite asymmetrically supported. These problems are less serious when cross-linked plastics
are used as embedding media, but at times continue to cause trouble,
particularly if unsupported sections are being studied. Thus the investigator must always be on the lookout for movement, and not be lulled
into a false sense of security.
When an intense (near cross-over) electron beam is being used there
can be field effects which act like weak lenses. More or less symmetrical
contaminating films on the objective aperture are most apt to do this.
These produce a shift of the focal level. Thus, if the investigator focuses
with a bright cross-over beam at high magnification, and then spreads out
the beam to take a picture with much less intensity, he may have lost
SPECIAL PHOTOGRAPHIC CONSIDERATIONS
307
"cold stage." These are really cold traps which prevent organic vapors
from reaching the specimen. Commercial designs have begun to appear
as accessories for certain microscopes. But their nuisance as well as their
cost make it unlikely that they will find favor for routine purposes.
Rapid photography becomes possible with knowledge. If one is
beginning a new investigation and needs time to get oriented, it is
best not even to attempt photography at first but use grids solely for
visual observation. There comes a time when the investigator has gained
confidence and has a good idea of what he wants to record on the film.
Then, as he comes to each new grid square, he should decide without
delay if anything is worthwhile recording. This decision must take
into account section thickness and contrast which will determine whether
high or low power pictures are feasible. No unnecessary time should be
wasted in focusing and taking the picture. However, since it is useless to
take an out-of-focus picture, all the time that is really needed to find
a suitable focus object, and to be sure of accurate focus should be used.
Just after focusing has been completed, one should pause and be certain
that there is no specimen drift which would ruin a picture. This is
particularly important when unstabilized plastic supporting films are
being used at relatively high magnification. They are then being subjected to an intense electron bombardment which produces strains as
differential decomposition occurs. A moving specimen ordinarily should
be abandoned, although very occasionally it will become stable if the
beam current is reduced. It is particularly dangerous with unstabilized
plastic supporting films to try to take pictures of squares which have the
film split across the square, or which have pulled away from one supporting wire. Such films almost invariably show movement. The great advantage of stabilizing plastic supporting films with evaporated carbon
is that the specimen may remain motionless even when quite asymmetrically supported. These problems are less serious when cross-linked plastics
are used as embedding media, but at times continue to cause trouble,
particularly if unsupported sections are being studied. Thus the investigator must always be on the lookout for movement, and not be lulled
into a false sense of security.
When an intense (near cross-over) electron beam is being used there
can be field effects which act like weak lenses. More or less symmetrical
contaminating films on the objective aperture are most apt to do this.
These produce a shift of the focal level. Thus, if the investigator focuses
with a bright cross-over beam at high magnification, and then spreads out
the beam to take a picture with much less intensity, he may have lost