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9.
PHOTOGRAPHY
focus. This effect is perfectly visible with the naked eye if one looks for it.
It is simplest to avoid the difficulty by focusing with the least possible
illumination, and not changing this much, if at all, to take pictures. One
can, of course, focus with a fairly bright illumination and then carefully
observe whether or not changes occur when the illumination is reduced
for picture taking. It is because of this potential danger of shifting the
focal plane when illumination is altered that electron microscopists have
come to take pictures at fairly high speeds in contrast to what was recommended originally. Most pictures are taken today at ^ or 1 second (rarely
over 2 seconds). This corresponds to just enough illumination to allow accurate visual focusing. It becomes desirable and almost necessary to have
an automatic shutter of some sort in the microscope. Moving the
fluorescent screen as a shutter is too cumbersome, for unless this is done
rapidly it is apt to expose one side of the plate proportionately more than
the other. The automatic shutter should be activated by an automatic
timer for working at these speeds.
The last thing one should do before taking a picture is to deliberately
examine the field to be sure that it is evenly illuminated. It is all too easy
to forget this, and to end up with a group of plates that have to be
elaborately "dodged" in the printing process to compensate for inequalities in the original illumination. If one is working close to cross-over
at high magnification it may, in fact, be quite difficult to find an adjustment of condenser strength and gun translation which gives truly even
illumination.
Anything which improves the visual brilliance of the viewing screen,
and so allows accurate focusing with reduced illumination, is advantageous. In some laboratories, the fluorescent screen is aluminized to produce a good reflecting surface, and then a very thin coat of fluorescent
material deposited on top of this surface. There is, thus, back reflection
within the fluorescent coat. The apparatus which is used for evaporating
carbon will serve for this sort of metal evaporation.
9.5. Reasons for Unsatisfactory Pictures
Good electron micrographs are beautifully sharp in their details and
have excellent contrast. If the beginner has trouble in achieving these
ends in many of his micrographs he should consider the various possible
sources of difficulty that begin when he first fixes the material, and end
9.
PHOTOGRAPHY
focus. This effect is perfectly visible with the naked eye if one looks for it.
It is simplest to avoid the difficulty by focusing with the least possible
illumination, and not changing this much, if at all, to take pictures. One
can, of course, focus with a fairly bright illumination and then carefully
observe whether or not changes occur when the illumination is reduced
for picture taking. It is because of this potential danger of shifting the
focal plane when illumination is altered that electron microscopists have
come to take pictures at fairly high speeds in contrast to what was recommended originally. Most pictures are taken today at ^ or 1 second (rarely
over 2 seconds). This corresponds to just enough illumination to allow accurate visual focusing. It becomes desirable and almost necessary to have
an automatic shutter of some sort in the microscope. Moving the
fluorescent screen as a shutter is too cumbersome, for unless this is done
rapidly it is apt to expose one side of the plate proportionately more than
the other. The automatic shutter should be activated by an automatic
timer for working at these speeds.
The last thing one should do before taking a picture is to deliberately
examine the field to be sure that it is evenly illuminated. It is all too easy
to forget this, and to end up with a group of plates that have to be
elaborately "dodged" in the printing process to compensate for inequalities in the original illumination. If one is working close to cross-over
at high magnification it may, in fact, be quite difficult to find an adjustment of condenser strength and gun translation which gives truly even
illumination.
Anything which improves the visual brilliance of the viewing screen,
and so allows accurate focusing with reduced illumination, is advantageous. In some laboratories, the fluorescent screen is aluminized to produce a good reflecting surface, and then a very thin coat of fluorescent
material deposited on top of this surface. There is, thus, back reflection
within the fluorescent coat. The apparatus which is used for evaporating
carbon will serve for this sort of metal evaporation.
9.5. Reasons for Unsatisfactory Pictures
Good electron micrographs are beautifully sharp in their details and
have excellent contrast. If the beginner has trouble in achieving these
ends in many of his micrographs he should consider the various possible
sources of difficulty that begin when he first fixes the material, and end