Abstract
When the title of the symposium was first decided, the organising committee considered that a general survey of the subject was required so that (a) they would be in a position to judge whether or not the papers received were a reasonable cross section of the subject, and (b) it could be used to indicate to the delegates where the subject of the individual speakers comes within the general picture. Unfortunately, there was none readily available, and therefore the author as a member of the organising committee, decided to formulate such a survey. Some parts of the survey are not as detailed or informative as intended, and the author would be grateful for advice on any errors or omissions which are noticed.
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