1908.] 



Anniversary Address by Lord Ray leigh. 



9 



that subject. It was there, too, amid the problems presented by the infinite 

 variety of tropical life, that he independently conceived the idea of the 

 theory of the origin of species by natural selection which Charles Darwin 

 had already been working out for years before. His claims to the admiration 

 of all men of science were recognised by the Eoyal Society forty years ago, 

 when, in 1868, a Eoyal Medal was awarded to him. Again, when in 1890, 

 the Darwin Medal was founded, he was chosen as its first recipient. He is 

 still full of mental activity and continues to enrich our literature with 

 contributions from his wide store of experience and reflection in the domain 

 of Natural History. As a crowning mark of the high estimation in which 

 the Eoyal Society holds his services to science, the Copley Medal is now 

 fittingly bestowed on him. 



Eumford Medal. 



The Eumford Medal is awarded to Prof. H. A. Lorentz, For. Mem. E.S. 



Prof. Hendrik Antoon Lorentz, of Leiden, has been distinguished during 

 the last quarter of a century by his fundamental investigations in the 

 principles of the theory of radiation, especially in its electric aspect. His 

 earliest memoirs were concerned with the molecular equivalents which 

 obtain in the refractive (and dispersive) powers of different substances ; 

 in them he arrived at formulae that still remain the accepted mode of 

 theoretical formulation of these phenomena. The main result, that 

 (/a 2 — l)/(fi 2 + 2) is proportional jointly to the density of distribution of the 

 molecules, and to a function of the molecular free periods and the period of 

 the radiation in question, rests essentially only on the idea of propagation 

 in some type of elastic medium ; and thus it was reached simultaneously, 

 along different special lines, by H. A. Lorentz originally from Helmholtz's 

 form of Maxwell's electric theory, and by L. Lorenz, of Copenhagen, from 

 a general idea of propagation after the manner of elastic solids. 



The other advance in physical science with which Prof. Lorentz's name 

 is most closely associated is one of greater precision, the molecular develop- 

 ment of Maxwell's theory of electro-dynamics. This subject was never 

 entered upon by Maxwell himself, on the ground, probably, that the general 

 relations of the sether, and in particular their dynamical bearings, offered 

 a definite field which must be fully probed and explored before the 

 uncertainties connected with molecular complexity became ripe for effective 

 detailed treatment. But the theoretical difficulties connected with the 

 simple law of the astronomical aberration of light, and particularly with 

 the entire absence of any effect of the Earth's uniform motion in space on 



