RECENT ADVANCES IN SCIENCE 627 



field and the mass of lucid stars can be estimated to a certain 

 degree of accuracy and their number approximately determined. 

 It will be seen, therefore, that Einstein's theory has scored 

 one signal success in giving an exact quantitative explanation 

 of the anomaly in the motion of the perihelion of Mercury, 

 for which the older theory of gravitation could not account. 

 The next test will doubtless be obtained from observations 

 of stars close to the limb of the sun, and a positive result to 

 this test will establish the theory on a very secure basis. 

 Gravitation has for so long resisted all attempts at explanation 

 or reduction to a theory, partly owing to the difficulty of testing 

 any theory, that it is with peculiar satisfaction that we con- 

 template the solution at last of the problem. It may be added 

 that, according to the theory, gravitation is propagated with 

 the velocity of light. 



References: Einstein und Grossmann, Zeitschr. fur Math. u. Physik, 1914, 

 Jan. ; Einstein, Sitsungsber. Berlin, 1914-16 (several papers) and Ann. der Physik, 

 xlix. p. 769, 1916; J. Droste, Proc. Acad. Sci. Amsterdam, xliii. p. 998, 1914 and 

 later ; Schwarzschild, Sitzungsber. Berlin, 1916, p. 189; de Sitter, Proc. Acad. Sci. 

 Amsterdam, xix. June and Sept. 1916, and M.N., R.A.S., lxxvi. p. 699, 1916, 

 lxxvii. p. 155, 1916, and Observatory, xxxix. p. 412, 1916. 



PHYSICS. By James Rice, M.A., The University, Liverpool. 



The Bakerian Lecture of last year was delivered by Prof. 

 Barkla, and is printed in the Philosophical Transactions. An 

 abstract of the address will be found in the Proc. Roy. Soc. 

 (A, 643, Aug. 1916). Dr. Barkla took as his subject " X-Rays 

 and the Theory of Radiation." The processes of emission of 

 X-rays may be considered under three heads : (1) Scattered 

 X-radiation, (2) Fluorescent or Characteristic X-radiation, and 

 (3) Primary X-radiation. Experiments on the quality, polarisa- 

 tion, distribution, and intensity of the scattered radiation from 

 any given substance show that it is emitted by electrons whose 

 motions, in so far as they are responsible for the scattered 

 radiation, are very completely controlled by the primary 

 radiation. Of late the quantum theory of radiation (viz. that 

 radiation is emitted discontinuously and in quantities which 

 are multiples of a definite unit and which make their exit from 

 the atom " catastrophically " when it is in a critical condition) 

 has received considerable support and has resolved some diffi- 

 culties attending the older view of continuous radiation. Now 



