292
8.
MICROSCOPY
Astigmatism can be seen most readily in diffraction images (Fresnel
fringes) at edges. Thus, small particles suspended in space make satisfactory specimens. Most simply, one takes a bit of filter paper dipped in
xylene, toluene, or benzene and ignites this in some safe place. A plume
of dense smoke arises, and a new specimen screen is held in this smoke
column until it is somewhat blackened, but not for such a length of
time that its holes are completely obliterated by the accumulated carbon.
Carbon particles obtained in this way are spherical except where they
adhere one to another, and are small enough so that they can be observed conveniently at high magnification. Thus, diffraction rings appear
fairly conspicuously when the image is slightly out of focus. These are
known as Fresnel fringes.
Small circular holes in fairly thick films are perhaps even better
objects for visualizing diffraction rings [Figs. 34(d)-(m)]. If one uses
unsupported sections or the method of section mounting that involves
evaporating carbon directly upon naked sections, one will have plenty
of small holes in almost any section screen. There will be no difficulty
finding and selecting suitable small holes which will serve for compensation. One can even put aside particularly favorable screens from time to
time, and keep them for compensation purposes. Thus, one is spared
the necessity of making special preparations.
Chapter 6.4 describes how plastic films full of small holes can be prepared, which are eminently suitable for compensation purposes if they
are given a relatively heavy coat of evaporated carbon. A group of grids
prepared in this way can be made at one time and stored indefinitely.
A single grid, in fact, can be used over and over again.
Compensation should be performed at the highest magnification
step that one expects to use. With some instruments, compensation may
be done at much higher magnifications, and it will be satisfactory in the
lower ranges as well. This will not necessarily follow, however, and one
should check this with his particular instrument and be sure that compensation is adequate over the whole range. It is necessary to use a
low power light microscope to see adequately the diffraction images on
the fluorescent screen.
The over-focused diffraction image has a conspicuous light band outlined by a dark line which is fairly easy to see [Fig. 34(d)]. With a wellcompensated pole-piece these bands circle the carbon particle or the rim
of the small hole. It is symmetrical in all axes [Fig. 34(d)]. The below-
8.
MICROSCOPY
Astigmatism can be seen most readily in diffraction images (Fresnel
fringes) at edges. Thus, small particles suspended in space make satisfactory specimens. Most simply, one takes a bit of filter paper dipped in
xylene, toluene, or benzene and ignites this in some safe place. A plume
of dense smoke arises, and a new specimen screen is held in this smoke
column until it is somewhat blackened, but not for such a length of
time that its holes are completely obliterated by the accumulated carbon.
Carbon particles obtained in this way are spherical except where they
adhere one to another, and are small enough so that they can be observed conveniently at high magnification. Thus, diffraction rings appear
fairly conspicuously when the image is slightly out of focus. These are
known as Fresnel fringes.
Small circular holes in fairly thick films are perhaps even better
objects for visualizing diffraction rings [Figs. 34(d)-(m)]. If one uses
unsupported sections or the method of section mounting that involves
evaporating carbon directly upon naked sections, one will have plenty
of small holes in almost any section screen. There will be no difficulty
finding and selecting suitable small holes which will serve for compensation. One can even put aside particularly favorable screens from time to
time, and keep them for compensation purposes. Thus, one is spared
the necessity of making special preparations.
Chapter 6.4 describes how plastic films full of small holes can be prepared, which are eminently suitable for compensation purposes if they
are given a relatively heavy coat of evaporated carbon. A group of grids
prepared in this way can be made at one time and stored indefinitely.
A single grid, in fact, can be used over and over again.
Compensation should be performed at the highest magnification
step that one expects to use. With some instruments, compensation may
be done at much higher magnifications, and it will be satisfactory in the
lower ranges as well. This will not necessarily follow, however, and one
should check this with his particular instrument and be sure that compensation is adequate over the whole range. It is necessary to use a
low power light microscope to see adequately the diffraction images on
the fluorescent screen.
The over-focused diffraction image has a conspicuous light band outlined by a dark line which is fairly easy to see [Fig. 34(d)]. With a wellcompensated pole-piece these bands circle the carbon particle or the rim
of the small hole. It is symmetrical in all axes [Fig. 34(d)]. The below-