Whether it is true or not that not more than twelve persons in all the world are able to understand Einstein's Theory, it is nevertheless a fact that there is a constant demand for information about this much-debated topic of relativity. The books pub- lished on the subject are so technical that only a person trained in pure physics and higher mathematics is able to fully understand them. In order to make a popular explanation of this far-reaching theory available, the present book is published. Professor Lorentz is credited by Einstein with sharing the development of his theory. He is doubtless better able than any other man-exceptthe author himself-to explain this scientific discovery. The publishers wish to acknowledge their indebtedness to the New York Times, The Review of Reviews andThe Athen- aeum for courteous permission to reprint articles from their pages. Professor Lorentz's article appeared originally in The Nieuwe Rotterdamsche Courant of November 19, 1919.