Whilst the greatest effort has been made to ensure the quality of this text, due to the historical nature of this content, in some rare cases there may be minor issues with legibility. It seemed best to leave practically untouched the account of the special principle as the natural avenue of approach to the broader view, which is described at length in Part II. Acknowledgment is due to the author's friends, Prof. H. F. Baker and Mr. E. V. Appleton, who kindly read through the proofs of the new matter.