10.6.
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337
if replicated in a standard way pose special problems. These materials
would include relatively large fibrous structures such as hair, and also
granular masses of substantial size. It is desirable then to replicate only a
part of the surface. Gelatin can be used to protect regions that subsequently will be replicated, while the rest of the specimen is embedded
in a plastic or a wax. A photographic plate which has been fixed and
washed, but which is still partly wet, provides a convenient flat, gelatin
layer upon which the specimen can be placed and, if necessary, slightly
pressed with a covering glass while the gelatin is allowed to dry. A cold
setting plastic or wax is then poured onto the plate embedding all parts
of the specimen except those previously embedded in the gelatin. Subsequently, the gelatin can be stripped away or dissolved easily, exposing
relatively flat portions of the specimen, which then can be replicated by
almost any of the standard techniques.
The images produced by all single-stage replicas are, of course, negatives of the original surface. However, this is not apt to cause serious
difficulty in their interpretation.
10.6. Preshadowed Replicas
If a shadowed single-stage replica is to be used, it is often best to metalshadow the surface to be replicated first, and then build the replica film
upon this. The metal will vigorously adhere to the replica, and usually
can be removed with it. Such replicas are usually called "preshadowed
replicas," or "pseudoreplicas." The resolution of such replicas is limited
only by the "grain" of the deposited metal.
In this procedure one ordinarily is concerned only with the image
produced by the metal-shadowing, and pays no attention to the relatively
faint image resulting from thickness variation of the replica. For this
reason the procedure more properly should be called a "shadow-transfer"
technique.
The method has found much application in virology and macromolecular research. It has an advantage over a single-stage replica, which
is shadowed after replication, in that the shadow images are positives.
Also, the angle of shadowing can be known exactly and employed quantitatively.
One interesting but highly specialized application of the method is the
technique developed by Steere (1957) in which frozen material is first
planed in vacuo, then etched slightly by vacuum sublimation, shadowed
PRESHADOWED REPLICAS
337
if replicated in a standard way pose special problems. These materials
would include relatively large fibrous structures such as hair, and also
granular masses of substantial size. It is desirable then to replicate only a
part of the surface. Gelatin can be used to protect regions that subsequently will be replicated, while the rest of the specimen is embedded
in a plastic or a wax. A photographic plate which has been fixed and
washed, but which is still partly wet, provides a convenient flat, gelatin
layer upon which the specimen can be placed and, if necessary, slightly
pressed with a covering glass while the gelatin is allowed to dry. A cold
setting plastic or wax is then poured onto the plate embedding all parts
of the specimen except those previously embedded in the gelatin. Subsequently, the gelatin can be stripped away or dissolved easily, exposing
relatively flat portions of the specimen, which then can be replicated by
almost any of the standard techniques.
The images produced by all single-stage replicas are, of course, negatives of the original surface. However, this is not apt to cause serious
difficulty in their interpretation.
10.6. Preshadowed Replicas
If a shadowed single-stage replica is to be used, it is often best to metalshadow the surface to be replicated first, and then build the replica film
upon this. The metal will vigorously adhere to the replica, and usually
can be removed with it. Such replicas are usually called "preshadowed
replicas," or "pseudoreplicas." The resolution of such replicas is limited
only by the "grain" of the deposited metal.
In this procedure one ordinarily is concerned only with the image
produced by the metal-shadowing, and pays no attention to the relatively
faint image resulting from thickness variation of the replica. For this
reason the procedure more properly should be called a "shadow-transfer"
technique.
The method has found much application in virology and macromolecular research. It has an advantage over a single-stage replica, which
is shadowed after replication, in that the shadow images are positives.
Also, the angle of shadowing can be known exactly and employed quantitatively.
One interesting but highly specialized application of the method is the
technique developed by Steere (1957) in which frozen material is first
planed in vacuo, then etched slightly by vacuum sublimation, shadowed