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6. SECTION MOUNTING
6.12, Mounting Screens in the Microscope Specimen
Holder
Most electron microscopes have a specimen holder with a cap into
which the specimen screen is placed. One soon becomes skillful at "flipping" screens into this. They are balanced on the edges of the cap and
then tipped in, turning over as they go.
Screens always should be mounted in the holder with the supporting
film and specimen down. It is important to standardize the orientation,
for magnification depends upon how close the specimen is to the polepiece of the objective lens. Calibration will be meaningless if the specimen is not always at the same level. It is preferable to have the specimen
down rather than up, to take advantage of as much magnification as possible. Also, in that position, the supporting film is in part protected from
the electron beam by the grid wires.
6.13. Dirty Specimens
At times dirt can so contaminate sections that they will be nearly useless. Unless the dirt comes with the specimen screens, it must be blamed
on the operator's faulty or careless technique. He has introduced the
dirt at one or another step during the sectioning, during the preparation
of supporting films, or during the final mounting. Perhaps the most dangerous times are during sectioning and when the sections are being picked
up. Dirty forceps tips, needles or spatulas may contaminate the fluid surface upon which the sections are floating. Oil films are the worst offenders. The trough itself or the plastic dish to which sections are transferred
may be contaminated in the first place, although there is no real excuse
for this if fingers are kept off the inner surfaces. The containers used to
hold the acetone and alcohol solution may be a source of contamination.
We have found that technical grade acetone may cause trouble and are
careful to use only reagent grade now. Also, demineralized water should
be employed in making up the solutions; otherwise fine crystalline precipitates may appear as contaminants. The water surfaces upon which
supporting films are floated are a dangerous source of contaminants as
indicated earlier in this chapter. Some "staining" operations are also
apt to lead to contaminated specimens, particularly since precipitates
6. SECTION MOUNTING
6.12, Mounting Screens in the Microscope Specimen
Holder
Most electron microscopes have a specimen holder with a cap into
which the specimen screen is placed. One soon becomes skillful at "flipping" screens into this. They are balanced on the edges of the cap and
then tipped in, turning over as they go.
Screens always should be mounted in the holder with the supporting
film and specimen down. It is important to standardize the orientation,
for magnification depends upon how close the specimen is to the polepiece of the objective lens. Calibration will be meaningless if the specimen is not always at the same level. It is preferable to have the specimen
down rather than up, to take advantage of as much magnification as possible. Also, in that position, the supporting film is in part protected from
the electron beam by the grid wires.
6.13. Dirty Specimens
At times dirt can so contaminate sections that they will be nearly useless. Unless the dirt comes with the specimen screens, it must be blamed
on the operator's faulty or careless technique. He has introduced the
dirt at one or another step during the sectioning, during the preparation
of supporting films, or during the final mounting. Perhaps the most dangerous times are during sectioning and when the sections are being picked
up. Dirty forceps tips, needles or spatulas may contaminate the fluid surface upon which the sections are floating. Oil films are the worst offenders. The trough itself or the plastic dish to which sections are transferred
may be contaminated in the first place, although there is no real excuse
for this if fingers are kept off the inner surfaces. The containers used to
hold the acetone and alcohol solution may be a source of contamination.
We have found that technical grade acetone may cause trouble and are
careful to use only reagent grade now. Also, demineralized water should
be employed in making up the solutions; otherwise fine crystalline precipitates may appear as contaminants. The water surfaces upon which
supporting films are floated are a dangerous source of contaminants as
indicated earlier in this chapter. Some "staining" operations are also
apt to lead to contaminated specimens, particularly since precipitates