It is unnecessary to stress the many significant contributions made during the past 20 years to nuclear and high-energy physics by means of the nuclear emulsion technique. One needs only to recall the new particles and decay modes that have been first observed with it. With the development of other powerful techniques, however, such as the spark-chamber and bubble-chamber, readily adaptable to automatic methods of analysis and data handling, nuclear emulsion has inevitably tended to fall into the position of a supplementary method. Nevertheless, there are still important experiments for which it is the most convenient, indeed in some cases the only, technique available, and this paper will discuss such experiments, either recently carried out or proposed for the future, using beams of particles from high-energy accelerators. Nuclear emulsion possesses one most significant advantage over all other techniques, namely, the extraordinarily high spatial resolution of which it is capable. Other techniques can resolve events separated by tenths of millimetres. Nuclear emulsion can resolve events separated by tenths of micrometres. This high spatial resolution has made possible the measurement of the lifetime of the
π
0
-meson (ca.10
-16
s) and is the basis of our confidence that there are no other commonly occurring unstable particles with lifetimes in the range 10
-11
to 10
-16
s. Most of the experiments described in this paper are particularly suited to the nuclear emulsion technique because they make use of this characteristic feature.
Nuclear Emulsion Technique
