Shortly after the inception of the Brown Boveri Research Center in 1966, plans were made to organize a series of biennial scientific symposia. A dif­ ferent subject was to be chosen for each symposium with the following re­ quirements in mind: It should characterize a part of a scientific discipline; in other words, it should describe an area of scholarly study and research. It should be of current interest in the sense that important results have recently been obtained and considerable research effort is under way in the world's scientific community. In other words, there must be a good reason why the symposium should be held