10.5.
SINGLE-STAGE REPLICATION
335
of surfaces, then, can be studied almost as though one were looking at the
originals.
Technical problems involved in making replicas, and particularly in
isolating them from the original specimens, are quite varied. They often
have been solved with much ingenuity. Thus this subject, even as it
relates to biological problems, has many facets, and it seems necessary
to subdivide it in the remaining sections of this chapter. The material
has been selected to present a few methods in sufficient detail to be useful, and to suggest the range of variation and application. Types of
replication and principles are discussed first, and finally replicating
materials and the practical problems of handing them are considered.
The investigator who requires a detailed review of methods of replication should consult Bradley (1961), who includes an extended bibliography covering industrial applications as well as biological.
10,5. Single-Stage Replication
If a replica is made in the form of a very thin film to begin with so
that it will be transparent to electrons, it can be examined directly in an
electron microscope. Such a replica is called a "single-stage" replica. If
it is prepared by flowing a solution of plastic on the surface to be replicated, its exposed upper surface will be smooth, the other surface contoured. The replica itself will be of variable thickness, and will produce
an electron image of varying contrast. Thus it can be used without
further ado, and under ideal conditions is capable of providing tremendous resolution. As the replica film is thickened, however, contrast is
quickly lost, and in most circumstances the replicated surface is observed
more conveniently after metal-shadowing (as discussed earlier in this
chapter). The contrast image produced by the shadowing is apt to overwhelm the minor density differences of the replica, and is the image
studied.
If a replica is made by the in vacuo evaporation of carbon, silica, or
some other light element, its exposed upper surface will be contoured
much like the replicated one, and the replica will be of nearly uniform
thickness. Such a replica must be metal-shadowed.
Inevitably, single-stage replicas are quite delicate objects. They are apt
to be difficult to isolate from the specimen unless the latter can be dissolved or digested away. If carbon is the replicating material, its chemical
SINGLE-STAGE REPLICATION
335
of surfaces, then, can be studied almost as though one were looking at the
originals.
Technical problems involved in making replicas, and particularly in
isolating them from the original specimens, are quite varied. They often
have been solved with much ingenuity. Thus this subject, even as it
relates to biological problems, has many facets, and it seems necessary
to subdivide it in the remaining sections of this chapter. The material
has been selected to present a few methods in sufficient detail to be useful, and to suggest the range of variation and application. Types of
replication and principles are discussed first, and finally replicating
materials and the practical problems of handing them are considered.
The investigator who requires a detailed review of methods of replication should consult Bradley (1961), who includes an extended bibliography covering industrial applications as well as biological.
10,5. Single-Stage Replication
If a replica is made in the form of a very thin film to begin with so
that it will be transparent to electrons, it can be examined directly in an
electron microscope. Such a replica is called a "single-stage" replica. If
it is prepared by flowing a solution of plastic on the surface to be replicated, its exposed upper surface will be smooth, the other surface contoured. The replica itself will be of variable thickness, and will produce
an electron image of varying contrast. Thus it can be used without
further ado, and under ideal conditions is capable of providing tremendous resolution. As the replica film is thickened, however, contrast is
quickly lost, and in most circumstances the replicated surface is observed
more conveniently after metal-shadowing (as discussed earlier in this
chapter). The contrast image produced by the shadowing is apt to overwhelm the minor density differences of the replica, and is the image
studied.
If a replica is made by the in vacuo evaporation of carbon, silica, or
some other light element, its exposed upper surface will be contoured
much like the replicated one, and the replica will be of nearly uniform
thickness. Such a replica must be metal-shadowed.
Inevitably, single-stage replicas are quite delicate objects. They are apt
to be difficult to isolate from the specimen unless the latter can be dissolved or digested away. If carbon is the replicating material, its chemical