306
9.
PHOTOGRAPHY
[Figs. 34(a)-(c)]. One then varies the focus with the fine control and
observes the changing complexity of the image. The image is in focus
when it shows the greatest variety of contrasts [Fig. 34(b)]. In the outof-focus images, contrast variation is lost so that the object comes to
look uniformly dense [Figs. 34(a) and (c)].
Even a very small and inconspicuous contaminant may give all the
information one needs for accurate focusing. In general, however, it will
be found that the piece of dirt which is adequate at low magnification
will not also serve at high magnification, and vice-versa. But it is the rare
square, indeed, that does not have imperfections that can be used with
reasonable success in this way. Occasionally, though, there may be no
imperfections to aid focusing, and one must resort to the specimen in
the section itself. The operator should then seek areas of greatest contrast. He will have to try to find a zone where there are reasonably
contrasting lines or particles that can be seen well enough with the
magnifying viewer to allow an estimate of focus. This may be difficult
or even impossible at high magnification. If the operator cannot be
reasonably sure of having achieved focus, there is no point in taking the
picture. There are occasions when the most experienced operators simply
have to abandon a grid square that they would like to photograph because they cannot find focus in it.
9.4. Special Considerations in Picture Taking
It is very important that the investigator learn to work fast in taking
pictures, for contaminating films quickly build up on specimens. This
tends to destroy resolution and quickly damages contrast. How quickly
depends partly on microscope design which cannot be altered, and importantly on how "clean" the column may be. There is general agreement
that oil vapors are the principal source of contamination. While some
of these come from the diffusion pump, excessively wide or thick films
associated with gaskets contribute significantly, and so do films from
handling internal parts. This is the most important reason for maintaining a clean column. Some laboratories go so far as to insist that
people wear nylon gloves whenever any internal part is touched, including particularly the specimen holder and the plate cassette. Most
people would regard this as more trouble than it is worth, but it cannot
be denied that the effort to avoid oil films contributes to improved performance. Parenthetically, it may be added that the only absolute solution to the contamination problem is a suitably designed and operated
9.
PHOTOGRAPHY
[Figs. 34(a)-(c)]. One then varies the focus with the fine control and
observes the changing complexity of the image. The image is in focus
when it shows the greatest variety of contrasts [Fig. 34(b)]. In the outof-focus images, contrast variation is lost so that the object comes to
look uniformly dense [Figs. 34(a) and (c)].
Even a very small and inconspicuous contaminant may give all the
information one needs for accurate focusing. In general, however, it will
be found that the piece of dirt which is adequate at low magnification
will not also serve at high magnification, and vice-versa. But it is the rare
square, indeed, that does not have imperfections that can be used with
reasonable success in this way. Occasionally, though, there may be no
imperfections to aid focusing, and one must resort to the specimen in
the section itself. The operator should then seek areas of greatest contrast. He will have to try to find a zone where there are reasonably
contrasting lines or particles that can be seen well enough with the
magnifying viewer to allow an estimate of focus. This may be difficult
or even impossible at high magnification. If the operator cannot be
reasonably sure of having achieved focus, there is no point in taking the
picture. There are occasions when the most experienced operators simply
have to abandon a grid square that they would like to photograph because they cannot find focus in it.
9.4. Special Considerations in Picture Taking
It is very important that the investigator learn to work fast in taking
pictures, for contaminating films quickly build up on specimens. This
tends to destroy resolution and quickly damages contrast. How quickly
depends partly on microscope design which cannot be altered, and importantly on how "clean" the column may be. There is general agreement
that oil vapors are the principal source of contamination. While some
of these come from the diffusion pump, excessively wide or thick films
associated with gaskets contribute significantly, and so do films from
handling internal parts. This is the most important reason for maintaining a clean column. Some laboratories go so far as to insist that
people wear nylon gloves whenever any internal part is touched, including particularly the specimen holder and the plate cassette. Most
people would regard this as more trouble than it is worth, but it cannot
be denied that the effort to avoid oil films contributes to improved performance. Parenthetically, it may be added that the only absolute solution to the contamination problem is a suitably designed and operated