GV-46 PHOTOGRAMMETRIC ENGINEERING 
chemical agents on the structure and properties of tooth enamel. At the same 
time, in the course of developing replica techniques, possible applications of the 
methods in the examination of other solids have been explored. The illustrations 
which follow have been selected from the records of these investigations to 
demonstrate the fine detail that can be reproduced in surface replicas and the 
pronounced tridimensional effect that is characteristic of shadowed prepara- 
tions. 
Two general types of replicas have been made from external tooth surfaces 
or from the surfaces of ground sections. Negative, or single stage, replicas have 
been prepared by applying a dilute solution of collodion to the surface and 
allowing it to dry. The film which forms is suitably thin for electron microscopy, 
but it is too fragile to be removed by manual stripping. In order to recover the 
replica the specimen must be dissolved away by immersion in acid (Ref. 5). 
Positive replicas have been made by a two-stage process (Ref. 6). A preliminary 
collodion replica, much too thick for electron microscopy, is stripped from the 
surface. The detail on this replica is then reproduced in a thin film of carbon, 
deposited vertically on the collodion through the evaporation of graphite in 
high vacuum. The resulting positive carbon replica is easily recovered by 
removing the collodion with suitable solvents. Positive replicas have the advan- 
tages that the specimens from which they are made are preserved, that much 
more irregular surfaces can be reproduced than with negative replicas, and that 
their thickness can be more rigidly controlled. As implied by the nomenclature, 
the elevations and depressions are inverted in negative replicas, whereas in 
positive replicas they are restored to their original perspective. 
Under certain conditions, the uppermost layer of a solid may become so 
firmly entrapped in a replica that it is actually removed with the film. When 
such an adherent layer is thin enough to be penetrated by the electron beam, 
it can often be examined profitably as part of the replica. Preparations of this 
type have come to be known as pseudo replicas (Ref. 1). They have had quite 
extensive application in studies of enamel and dentin. 
Some examples of the structural detail revealed in replicas of the etched 
surfaces of ground sections of teeth are presented in Figure 1. Much that is 
new has been learned from such preparations about both the inorganic and 
organic components of enamel and dentin, many of which are so small that 
they can be seen only under the electron microscope. Pseudo-replicas of enamel, 
like the one shown in Figure 1A, have been particularly instructive, for the 
crystallites actually present can be observed and measured directly under the 
electron microscope. In addition, they can be identified as apatite (tricalcium 
phosphate) by subjecting the pseudoreplicas to electron diffraction. The 
marked tridimensional appearance of micrographs made from replicas of dentin 
is illustrated in Figure 1B-D. The varying heights of the elevations in these 
pictures are indicated clearly by the differences in the lengths of the shadows. 
A few of the other types of surfaces which may be studied by means of replicas 
are depicted in Figure 2. 
Replica techniques have proved very useful in studies of the physical and 
chemical properties of tooth enamel. Many of the experiments have been di- 
rected toward evaluating the effectiveness of various chemical agents in increas- 
ing the resistance of the outer enamel surface to acid action. Although the exact 
mechanisms through which enamel is destroyed in dental caries are still not 
clearly understood, there is considerable evidence that acid action may be a 
major factor. The ability to increase acid resistance of enamel is thus one of 
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