Before beginning this lecture I have first of all to thank the Air Ministry for permission to give it and to explain that any opinions expressed are my own and do not necessarily reflect the opinion of the Air Ministry.

Secondly I must express my appreciation of the honour conferred on me by the Society in asking me to deliver a lecture before them; though I wish they had chosen a less difficult subject.

As regards the title of this lecture it sounds very circumscribed, but you can't test anything without having a fair idea of what its complete function is or without some standard of reference; and in trying to discuss these parts of the problem I am afraid you will find that I have been, if anything, too. general.