332
10.
PARTICULATE SPECIMENS
are shielded and receive no deposit. This is the "shadow" which in a
positive print appears white. However, in a negative print the shadows
appear black, while the metalized surfaces are light. Thus, negative prints
do indeed look like pictures of contoured objects brilliantly illuminated
from the side by light (Figs. 41 and 9), and ordinarily are used. Obviously,
if one knows the angle of shadowing, 0, and the length of the shadow, I,
the height of the object, h, which produced the shadow can be determined
by simple trigonometry (/* = /• tan 0). In this way the third dimension of
objects can be ascertained.
Shadowing is of great importance whenever small objects of relatively
low density must be examined, for the contours of such objects may not
be sharply defined otherwise. Furthermore, of course, fine order surface
detail may be revealed which could not be visualized in any other way.
Metal evaporation is done in vacuo. The system must be capable of a
good vacuum if sharp shadows are to be obtained. The vacuum chamber
should be readily accessible (usually a bell jar), and of considerable size
(perhaps 10-12 in. in diameter and height) to allow flexible operation.
Heavy gauge (approximately 15 mil) tungsten wire heated electrically
vaporizes the metal used as the shadowing agent. For this one wants a
low-voltage high-amperage system. A very heavy duty 6, 8, or 12-volt
transformer, which can handle currents approaching 100 amps (at least
50 amps), is required. Its output can be regulated by varying the input.
Thus a variable transformer is used in the primary circuit.
The substance to be evaporated may be supplied in granular or even
powder form, or be wire, ribbon, or foil. The tungsten heating wire can
be adapted to accommodate any of these. For granules or powders small
baskets are made by winding the tungsten wire spirally around the end
of a pencil. If the metal comes in wire, ribbon, or foil form, it can be
wrapped or clamped over the tungsten wire at a region where the tungsten
wire is bent to a V-shaped point. The tungsten wire will heat most intensely where it has been kinked, and the molten evaporating metal will
form a liquid drop at the base of the V.
A number of metals and a few other atoms can be used for shadowing.
Chromium and gold have low boiling points and thus are easy to use.
Chromium, however, is a fairly light element and therefore relatively
thick metal layers must be built up to produce high contrast. This does
not matter at moderate magnifications, and its convenience recommends
it (Figs. 41 and 9), but it becomes a serious drawback if one is concerned
with very fine order detail at high resolution. Also, partly because of the
10.
PARTICULATE SPECIMENS
are shielded and receive no deposit. This is the "shadow" which in a
positive print appears white. However, in a negative print the shadows
appear black, while the metalized surfaces are light. Thus, negative prints
do indeed look like pictures of contoured objects brilliantly illuminated
from the side by light (Figs. 41 and 9), and ordinarily are used. Obviously,
if one knows the angle of shadowing, 0, and the length of the shadow, I,
the height of the object, h, which produced the shadow can be determined
by simple trigonometry (/* = /• tan 0). In this way the third dimension of
objects can be ascertained.
Shadowing is of great importance whenever small objects of relatively
low density must be examined, for the contours of such objects may not
be sharply defined otherwise. Furthermore, of course, fine order surface
detail may be revealed which could not be visualized in any other way.
Metal evaporation is done in vacuo. The system must be capable of a
good vacuum if sharp shadows are to be obtained. The vacuum chamber
should be readily accessible (usually a bell jar), and of considerable size
(perhaps 10-12 in. in diameter and height) to allow flexible operation.
Heavy gauge (approximately 15 mil) tungsten wire heated electrically
vaporizes the metal used as the shadowing agent. For this one wants a
low-voltage high-amperage system. A very heavy duty 6, 8, or 12-volt
transformer, which can handle currents approaching 100 amps (at least
50 amps), is required. Its output can be regulated by varying the input.
Thus a variable transformer is used in the primary circuit.
The substance to be evaporated may be supplied in granular or even
powder form, or be wire, ribbon, or foil. The tungsten heating wire can
be adapted to accommodate any of these. For granules or powders small
baskets are made by winding the tungsten wire spirally around the end
of a pencil. If the metal comes in wire, ribbon, or foil form, it can be
wrapped or clamped over the tungsten wire at a region where the tungsten
wire is bent to a V-shaped point. The tungsten wire will heat most intensely where it has been kinked, and the molten evaporating metal will
form a liquid drop at the base of the V.
A number of metals and a few other atoms can be used for shadowing.
Chromium and gold have low boiling points and thus are easy to use.
Chromium, however, is a fairly light element and therefore relatively
thick metal layers must be built up to produce high contrast. This does
not matter at moderate magnifications, and its convenience recommends
it (Figs. 41 and 9), but it becomes a serious drawback if one is concerned
with very fine order detail at high resolution. Also, partly because of the