The difference between the laws of gravitation of Einstein and Newton come only in special cases. The real interest of Einstein's theory lies not so much in his results as in the method by which he gets them. If his theory is right, it makes us take an entirely new view of gravitation. If it is sustained that Einstein's reasoning holds good--and it has survived two very severe tests in connection with the perihelion of mercury and the present eclipse--then it is the result of one of the highest achievements of human thought. The weak point in the theory is the great difficulty in expressing it. It would seem that no one can understand the new law of gravitation without a thorough knowledge of the theory of invariants and of the calculus of variations. One other point of physical interest arises from the discussion. Light is deflected in passing near huge bodies of matter. This involves alterations in the electric and magnetic field. This, again, implies the existence of electric and magnetic forces outside matter--forces at present unknown, though some idea of their nature may be got from the results of this expedition. NOTES [1] See Note 1 at the end of the volume. [2] See Note 2. [3] See Note 3. [4] A circle--in our case the horizon--is measured by dividing the circumference into 360 parts; each part is called a degree. Each degree is divided into 60 minutes, and each minute into 60 seconds. [5] See page 113. [6] See Note 4. [7] See Note 5. [8] See Note 6. [9] See Note 7. [10] See Note 8. [11] See Note 9. [12] See page 93. [13] This has since been translated into English by Dr. Lawson and published by Methuen (London). Since the above has been written two excellent books have been published. One is by Prof. A. S. Eddington, Space, Time and Gravitation (Cambridge Univ. Press, 1920). The other, somewhat more of a philosophical work, is Prof. Moritz Schlick's Space and Time in Contemporary Physics (Oxford Univ. Press, 1920). Though published as early as 1897, Bertrand Russell's An Essay on the Foundations of Geometry (Cambridge Univ. Press, 1897) contains a fine account of non-Euclidean geometry. [14] Republished by permission from "Science." [15] Presidential address delivered at the St. Louis meeting of the Physical Society, December 30, 1919. Republished by permission from "Science." [16] From a report in The Observatory, of the Joint Eclipse Meeting of the Royal Society and the Royal Astronomical Society, November 6, 1919. End of Project Gutenberg's From Newton to Einstein, by Benjamin Harrow