But if we will not have this theory we must reject the earlier supposition that the velocity of light _in vacuo_ is the same in all directions. This we do, in fact, by assuming an ether, and assuming a certain law for its modification. Then we, in fact, adopt the first supposition so far as to hold that there are dynamical axes specially at rest, namely, at rest relatively to the undisturbed ether. Then an assumed law for the modification of the ether so alters the velocity of light that we explain why no dynamical axes show traces of motion. I wish now to go back to the point which I made a few minutes ago, that what we perceive at any instant is ancient history with its dates hopelessly mixed. In the earlier part of my comments I emphasised the point that our only data as to the physical world are our sensible perceptions. We must not slip into the fallacy of assuming that we are comparing a given world with given perceptions of it. The physical world is in some general sense of the term a deduced concept. Our problem is, in fact, to fit the world to our perceptions, and not our perceptions to the world. PRINTED IN GREAT BRITAIN BY RICHARD CLAY & SONS, LIMITED, BRUNSWICK ST., STAMFORD ST., S.E., AND BUNGAY, SUFFOLK. From WILLIAMS & NORGATE'S LIST =A SPIRITUAL PILGRIMAGE.= By the Rev. R. J. CAMPBELL. (The Rev. R. J. Campbell's own Story of his Religious Life.) 2nd Impression. Demy 8vo. =7s. 6d.= net. "This is eminently a book for which to be thankful--simple, straightforward, kindly. Neither does it lack the saving grace of humour."--_Guardian._ "Mr. Campbell devotes a chapter to his re-ordination, and it must be admitted that his defence of this act is effective."--_Methodist Times._ =EDINBURGH.= By the Right Hon. Sir HERBERT MAXWELL, Bart., D.C.L., Author of "Life of the Duke of Wellington," "Scottish Gardens," etc. President of the Society of Antiquaries of Scotland, 1910-13, and Chairman of the Royal Commission on Scottish Historical Monuments. With a Coloured Frontispiece and 64 pages of Illustrations of the past and present city. Medium 8vo. =10s. 6d.= net. A History of the Scottish Capital from the earliest times to the nineteenth century. =FURTHER PAGES OF MY LIFE.= By the Right Rev. W. BOYD CARPENTER, K.C.V.O., D.D., D.C.L., Formerly Bishop of Ripon, Author of "Some Pages of My Life," "The Witness of Religious Experience," etc. With Illustrations. Medium 8vo. =10s. 6d.= net. Reflections mingled with intimate reminiscences and recollections of Royal personages and of men eminent in many spheres. =SOME PAGES OF MY LIFE.= By the Right Rev. W. BOYD CARPENTER. New and Cheaper Edition. Large post 8vo. =5s.= net. All who have listened to his eloquent preaching will read with delight his musing on what life has brought the author.