9.3.
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305
is outlined by a light band which gives heightened contrast to the edge,
but since the background itself is light, this band is not conspicuous
[Figs. 34(g) and (h) and 40(c) and (d)]. The truly in-focus image shows
no diffraction bands, and the edge therefore is without special contrast
emphasis [Figs. 34(e) and (f) and 40(b)]. If one examines an edge with
high magnification and adjusts the fine focus control so that one goes
back and forth through focus, it will be seen that at true focus the edge
is least conspicuous. It is not difficult to approximate this by starting
with the overfocused band plainly visible, and reducing the lens current
until this just disappears. However, if the novice will take the trouble
to make a number of test negatives he will soon realize that his eye is
not really good enough to focus exactly. If true focus pictures must be
obtained for very high resolution studies, a through-focus series of pictures is almost necessary. Fortunately, for most work, very slightly outof-focus pictures are actually ordinarily preferable because of their
heightened contrast; so that, in practice, one can rely on the
fluorescent
image for adequate focusing (Fig. 40).
Naturally, diffraction bands will not be particularly evident if there
are not suitable contrast conditions at the edge being observed. Thus,
a hole in a very thin and transparent supporting film may prove inadequate. On the other hand, a similar edge in a section area where the
density is greater can be expected to produce an easily visible diffraction
band. If the edge of an object on top of a supporting film is being examined it probably will be necessary that the object have great density
if a diffraction band is to be seen clearly. Thus, a fairly large, opaque
piece of dirt may do, but a smaller, more transparent piece be inadequate.
A fold in the section often will suffice. Ordinarily the edge of a tear in
a section that lies over an intact supporting film will not have sufficient
contrast.
There will be times when the operator cannot use diffraction bands
with high contrast edges for focusing. A square being studied at high
magnification may not offer suitable opportunity for this type of focusing
or, at low magnifications, the diffraction bands may be too narrow to be
seen adequately. Then one must resort to another trick that ultimately
also depends upon diffraction phenomena. One seeks out a piece of dirt,
a small fold, or perhaps a thick ragged spot in the section. The object
selected must show a range of contrasts. It must not be totally opaque
or of a uniform grayness. It should have at least two distinct density
values of its own which contrast with the relatively bright background
FOCUS
305
is outlined by a light band which gives heightened contrast to the edge,
but since the background itself is light, this band is not conspicuous
[Figs. 34(g) and (h) and 40(c) and (d)]. The truly in-focus image shows
no diffraction bands, and the edge therefore is without special contrast
emphasis [Figs. 34(e) and (f) and 40(b)]. If one examines an edge with
high magnification and adjusts the fine focus control so that one goes
back and forth through focus, it will be seen that at true focus the edge
is least conspicuous. It is not difficult to approximate this by starting
with the overfocused band plainly visible, and reducing the lens current
until this just disappears. However, if the novice will take the trouble
to make a number of test negatives he will soon realize that his eye is
not really good enough to focus exactly. If true focus pictures must be
obtained for very high resolution studies, a through-focus series of pictures is almost necessary. Fortunately, for most work, very slightly outof-focus pictures are actually ordinarily preferable because of their
heightened contrast; so that, in practice, one can rely on the
fluorescent
image for adequate focusing (Fig. 40).
Naturally, diffraction bands will not be particularly evident if there
are not suitable contrast conditions at the edge being observed. Thus,
a hole in a very thin and transparent supporting film may prove inadequate. On the other hand, a similar edge in a section area where the
density is greater can be expected to produce an easily visible diffraction
band. If the edge of an object on top of a supporting film is being examined it probably will be necessary that the object have great density
if a diffraction band is to be seen clearly. Thus, a fairly large, opaque
piece of dirt may do, but a smaller, more transparent piece be inadequate.
A fold in the section often will suffice. Ordinarily the edge of a tear in
a section that lies over an intact supporting film will not have sufficient
contrast.
There will be times when the operator cannot use diffraction bands
with high contrast edges for focusing. A square being studied at high
magnification may not offer suitable opportunity for this type of focusing
or, at low magnifications, the diffraction bands may be too narrow to be
seen adequately. Then one must resort to another trick that ultimately
also depends upon diffraction phenomena. One seeks out a piece of dirt,
a small fold, or perhaps a thick ragged spot in the section. The object
selected must show a range of contrasts. It must not be totally opaque
or of a uniform grayness. It should have at least two distinct density
values of its own which contrast with the relatively bright background