304
9.
PHOTOGRAPHY
focus). The heightened contrast of the out-of-focus image results from
diffraction phenomena (Fresnel fringes) which are minimized in the true
focal plane (Fig. 40). The beginner must learn to focus intellectually,
following rules rather than relying on his visual impression. The situation, of course, is slightly complicated because a fluorescent screen does
not provide a truly sharp image. The photographic plate can be expected to show a wealth of detail not visible on the fluorescent screen.
In essence we are able to record on a photographic plate detail that we
cannot see to focus. It is this resolution that one strives for in making
a picture.
It is foolish to attempt to take a picture if there is any doubt about
the focus. One must resort to every possible aid. It is essential that one
have magnification for viewing the fluorescent screen. This should correspond to the intrinsic resolving power of the screen, which will be
about X7-10. Binocular "dissecting" microscope bodies are ideal in this
respect.
It is difficult, at best, to focus on the part of the image that one ultimately wants recorded. A part of a section that is thin enough to be
useful finally is a very unsatisfactory object, for its contrast on the
fluorescent screen is altogether too low. One needs a piece of dirt or
a sharp edge for adequate contrast. Fortunately the electron microscope
has relatively great depth of focus, so that if one can focus sharply on
any part of a grid square, one then can move about within that square,
and take a sharp picture of the desirable part of the specimen. It is the
rare square indeed which does not have some little blemish that can be
used for focusing.
The most accurate focusing of all can be done when diffraction bands
can be observed at a sharp edge. For this purpose a hole or a tear in the
section is ideal (Figs. 34 and 40). The edge of a fold in the section or of
an opaque particle may serve also. Some investigators deliberately spray
a few latex particles on sections just so they can use their edges for
focus. Out-of-focus diffraction bands are intrinsically narrow, however,
so that this method is useful only at the middle and upper ranges of
magnification. At low magnifications suitably chosen dirt particles serve
best [Figs. 34(a)-(c)].
When out-of-focus diffraction bands can be seen, it is the overfocused
band which is most conspicuous. Then the edge seemingly is outlined
by a light line followed by a dark one; it is the light band that is
particularly conspicuous [Figs. 34(d) and 40(a)]. Below focus, the edge
9.
PHOTOGRAPHY
focus). The heightened contrast of the out-of-focus image results from
diffraction phenomena (Fresnel fringes) which are minimized in the true
focal plane (Fig. 40). The beginner must learn to focus intellectually,
following rules rather than relying on his visual impression. The situation, of course, is slightly complicated because a fluorescent screen does
not provide a truly sharp image. The photographic plate can be expected to show a wealth of detail not visible on the fluorescent screen.
In essence we are able to record on a photographic plate detail that we
cannot see to focus. It is this resolution that one strives for in making
a picture.
It is foolish to attempt to take a picture if there is any doubt about
the focus. One must resort to every possible aid. It is essential that one
have magnification for viewing the fluorescent screen. This should correspond to the intrinsic resolving power of the screen, which will be
about X7-10. Binocular "dissecting" microscope bodies are ideal in this
respect.
It is difficult, at best, to focus on the part of the image that one ultimately wants recorded. A part of a section that is thin enough to be
useful finally is a very unsatisfactory object, for its contrast on the
fluorescent screen is altogether too low. One needs a piece of dirt or
a sharp edge for adequate contrast. Fortunately the electron microscope
has relatively great depth of focus, so that if one can focus sharply on
any part of a grid square, one then can move about within that square,
and take a sharp picture of the desirable part of the specimen. It is the
rare square indeed which does not have some little blemish that can be
used for focusing.
The most accurate focusing of all can be done when diffraction bands
can be observed at a sharp edge. For this purpose a hole or a tear in the
section is ideal (Figs. 34 and 40). The edge of a fold in the section or of
an opaque particle may serve also. Some investigators deliberately spray
a few latex particles on sections just so they can use their edges for
focus. Out-of-focus diffraction bands are intrinsically narrow, however,
so that this method is useful only at the middle and upper ranges of
magnification. At low magnifications suitably chosen dirt particles serve
best [Figs. 34(a)-(c)].
When out-of-focus diffraction bands can be seen, it is the overfocused
band which is most conspicuous. Then the edge seemingly is outlined
by a light line followed by a dark one; it is the light band that is
particularly conspicuous [Figs. 34(d) and 40(a)]. Below focus, the edge