NATURE 



361 



THURSDAY, AUGUST 18, 1904. 



SCIENTIFIC WORTHIES. 



XXXIV. — Lord Rayleigh. 



f T ORD RAYLEIGH 'S investigations have covered 



•L' so wide a field, and the advances he has made 

 in every direction have been so marked, that it is no 

 easy task to give an account of them in a short notice. 



He has left his impress on all branches of physical 

 research, an impress distinguished by the thoroughness 

 and completeness of his work, by his skilful adapt- 

 ation of means to ends, by a keen insight into the 

 essentials of a problem, and by a strong grasp of the 

 fundamental principles required for its solution. 



Whether it be in his early work before the days 

 when he held the Cavendish professorship, in the 

 series of electrical measurements that marked his 

 short tenure of that post, or in the discoveries made 

 more recently in his own laboratory at Terling, these 

 distinctive features stand out; the simplest means are 

 selected, the devices nearest to hand adopted, but the 

 whole procedure is directed to solving, as accurately 

 as the conditions of the work will allow, the problem 

 in view. 



The four splendid volumes recently issued by the 

 Cambridge L'niversity Press are a striking monument 

 of his ability; these, with the "Theory of Sound," 

 contain his contributions to the advancement of natural 

 knowledge, and with their help it becomes possible to 

 give some idea of his work. 



Born on November 12, 1842, the son of the second 

 Lord Ravleigh, he was educated at Trinity College, 

 Cambridge, which he entered in October, 1861. 



Some three years later, January, 1865, he took his 

 degree as senior wrangler. In 1866 he was elected to 

 a fellowship, which he held until his marriage in 1871. 



The first paper in his collected works is dated 

 i86g. His latest contribution to science was read 

 before the Mathematical Society in June of the present 

 year, and deals with Poisson's solution of the differ- 

 ential equation of wave motion. 



The earlier papers are miscellaneous in character. 

 One of the most important, on the theory of resonance 

 iPhil. Trans., 1870), appears again in an altered form 

 as an important section of the "Theory of Sound," 

 while a series of optical papers in the Philosophical 

 Magasine for 187 1, dealing with the colour of the sky, 

 the scattering of light by small particles, double re- 

 fraction, and the reflection of light from transparent 

 matter, have long been classical. 



In the first of these he proved that the intensity of 

 ) the light reflected from small particles varies inversely 



as the fourth power of the wave-length; the third 

 paper discusses the properties of an incompressible 

 ;Eolotropic ethei, in which the inertia is a function of 

 the direction, while the forces are the same as those 

 in an isotropic medium; it appears that in such an 

 NO. 1 81 6, VOL. 70J 



ether the wave surface would not be that of Fresnel ; 

 it has since been shown that if the medium be not 

 incompressible, but be such that the forces resisting 

 compression are zero, then the wave surface is 

 Fresnel's, but the motion is no longer in the wave 

 front. The fourth paper of this series puts in a clear 

 and definite manner the strict theory of reflection of 

 waves in an elastic solid as developed by Green, dis- 

 tinguishing clearly between his results and those of 

 other writers less rigid in their demonstrations. 



Another series of optical papers of great importance 

 was published in the Philosophical Magazine nine 

 years later, in 1880. 



These dealt with the theory of certain optical instru- 

 ments, especially with regard to their separating or 

 resolving power. In the first of these is established 

 the fundamental law that a double line, when viewed 

 through a telescope, cannot be fairly resolved unless its 

 components subtend an angle exceeding that subtended 

 by the wave-length of light at a distance equal to the 

 horizontal aperture, and the principles leading to this 

 law, applicable to a telescope, have been extended to 

 the case of a microscope in more recent papers pub- 

 lished in the Philosophical Magazine in 1896, and in 

 the Journal of the Royal Microscopical Society, 1903. 



Lord Rayleigh 's contributions to optics are, however, 

 best summed up in his article on wave theory in the 

 ninth edition of the " Encyclopaedia Britannica," and 

 in themselves constitute a claim to the highest distinc- 

 tion. 



Clerk Maxwell died in the autumn of 1879, and Lord 

 Rayleigh, in response to an influential memorial, ex- 

 pressed his willingness to carry on the work at the 

 Cavendish Laboratory. He was of course elected, and 

 for the next five years devoted himself to the duties of 

 the chair. The collected papers show us how fruitful 

 those years were ; moreover, in addition to original 

 research, there was much to be done in organising the 

 teaching and practical work in the laboratory. On 

 his appointment pupils were few, and organised in- 

 struction hardly existed. Maxwell had collected a 

 number of graduate students and inspired them with 

 a keen love for research and investigation, but the 

 practical classes which in Lord Rayleigh 's time became 

 so prominent a feature of the laboratory had almost 

 to be formed. In 1879 ''^^ natural science tripos con- 

 tained twenty-five names ; in 1884 there were eighty 

 successful candidates, very many of whom obtained 

 a large portion of their training at the laboratory. 



The researches of this period include the well known 

 series of investigations into the fundamental electrical 

 units. These were determined with an accuracy which 

 it is difficult even after twenty years' experience of 

 similar measurements to surpass, and have formed 

 the foundation for the legal standards of resistance, 

 current, and electromotive force throughout the 

 world. 



Allusion should be made to a short paper in the Philo- 

 sophical Magazine for 1892 on a comparison of the 

 methods for the determination of resistance in absolute 



