PREFACE TO SECOND EDITION The purpose of this edition is not to change in any way the substance of the original recommendations but only to re-state them in a manner which renders them free from ambiguity or the possibility of misconstruction. The recommendations as given in this report have been approved by the Directors of the National Laboratories of London@ Paris and Washington. Through the good offices of the President of the Swiss Committee this revised report has been reviewed by Prof. E. Warburg.