THE theory of dislocations was introduced in order to explain the plas- ticity and work hardening of crystals} and it may seem anomalous that
a long book on dislocations in crystals should contain little discussion
of their influence on the process of plastic deformation and of the way
in which this process leads to the formation of complex arrays of dis-
locations. Although much progress has been made and methods for the
direct observation of dislocations are now highly developed,2 there is at
present no generally agreed theory of work hardening?’ It is even difficult
to discuss empirical generalizations on plasticity and brittleness such as
those of Pugh4 and Bell,5 but progress is being made in this direction.“
We may say that a crystal is ductile rather than brittle when Poisson’s
ratio is large because the edge dislocations are then wide and the
Peierls force resisting dislocation motion is small. We might alter-
natively say that a solid with a large Poisson ratio resists changes of shape
much less than it resists changes of volume, but our description using
dislocations displays the mechanism by which this underlying tendency
is able to operate. Unfortunately, it can be given only in very general
terms. I have therefore concentrated on the parts of dislocation theory
which seem to have reached a degree of stability. I have made an
attempt to trace ideas back to their historical origins. In some parts,
such as the elastic theory of dislocations, it has been possible to cover
the present state of knowledge fairly completely. Elsewhere, as in the
study of dislocations in particular crystal structures or in the theory
of the interaction of dislocations with point defects, this would hardly
be possible.