202 



NATURE 



{July 15, 1875 



drudgery of Secretary to tjie Commissioners for the 

 University of Cambridge ; or of a Lecturer in the School 

 of Mines ; or the exhausting labour and totally inadequate 

 remuneration of a Secretary to the Royal Society ! Men 

 know about these things, as well as about a good many 

 other important things, much better in Germany than we 

 yet know them ; and it will not be very long before we 

 in our turn will be forced to know them to the full as well. 

 Let us hope that this knowledge may come to us in a more 

 gentle form than that of the rude and sudden lessons 

 which have so lately been read (for something very like 

 the same fatal blindness) alike to Austria and to France ! 



The magnificent Royal Society Catalogue of Scientific 

 Papers^ one of the greatest boons ever conferred on men 

 of science, shows that up to 1864 Stokes had pubhshed 

 the results of some seventy distinct investigations ; on an 

 average between three^and four per annum. Several of 

 these are controversial ; designed not so, much to establish 

 new results as to upset false and dangerously misleading 

 assertions. Some are improvements on the mathematical 

 methods usually employed in the treatment of compara- 

 tively elementary portions of physics ; and, especially 

 those on the Hydrokineiic Equations and on Waves, are 

 exceedingly valuable. These appeared in the Cambridge 

 and Dublin Mathematical Journal. 



Of the higher purely mathematical papers of Stokes we 

 cannot here attempt to give even a meagre sketch. It 

 would be hopeless to attempt to give the general reader 

 an idea of what is meant by- the " Critical Values of the 

 Sums of Periodic Series," or even by the "Numerical 

 Calculation of Definite Integrals and Infinite Series;" 

 though we may simply state that under these heads are 

 included some of the most important improvements which 

 pure mathematics have recently received with the view of 

 fitting them for physical applications. 



In applied mathematics it is hard to make a selection, 

 so numerous and so important are Stokes' papers. But 

 we may mention specially the following :— 



" On the Friction of Fluids in Motion, and the Equili. 

 brium and Motion of Elastic Solids." Camb. Phil, 

 Trans., 1845. 



" On the Effects of the Internal Friction of Fluids oa 

 the Motion of Pendulums." Ibid. 1850. 



(In these papers, for the first time, it is shown how to 

 take account of difference of pressure in different direc- 

 tions in the equations of motion of a viscous fluid ; the 

 suspension of globules of water in the air as a cloud is 

 for the first time explained and the vesicular theory 

 utterly exploded ; and the notion of Navier and Poisson 

 as to a necessary numerical relation between the rigidity 

 and the compressibility of a solid is shown to be un- 

 tenable. Each one of these is a distinct, and exceedingly 

 great, advance in science ; but they are only single gems 

 chosen, as we happen to recollect them, from z. rich 

 treasury.) 



Then we have a series of magnificent researches on the 

 ** Undulatory Theory of Light," for the most part also 

 published in the Cambridge Philosophical Transactions. 

 Of these we need mention only three : — 



" On the Dynamical Theory of Diffraction." 1849. 



(Here, in addition to "a splendid experimental inquiry 

 as to the position of the plane of polarisation with refer- 

 ence to the direction of vibration, we have an invaluable 

 inquiry into the properties and relations of Laplace's 

 ' 'i'ei-ator, an inquiry bearing not alone upon the Undula- 



tory Theory, but also upon gravity, electric and magnetic 

 attractions, and generally upon all forces whose intensity 

 is inversely as the square of the distance.) 



" On the Colours of Thick Plates." 185 1. 



*' On the Formation of the Central Spot of Newton's 

 Rings beyond the Critical Angle." 1848. 



As another most important contribution to the undu- 

 latory theory we have his 



" Report on Double Refraction." Biitish Association 

 Report, 1862. 



Then we have a full investigation, in one respect carried 

 to a third approximation, of the propagation of waves in 

 water ; a complete explanation of the extremely rapid sub- 

 sidence of ripples by fluid friction, &c. 



Another paper of great value is — 



" On the Variation of Gravity at the Surface of th« 

 Earth." Camb. Phil. Trans., 1849. 



Perhaps Stokes is popularly best known by his experi- 

 mental explanation of Pluorescence. This is contained in 

 his paper 



" On the Changel^of the Refrangibility of Light." Phil. 

 Trans., 1852. 



There can be no doubt, as was well shown by Sir W. 

 Thomson in his Presidential Address to the British Asso- 

 ciation at Edinburgh in 1871, that Stokes (at least as early 

 as 1852) had fully apprehended the physical basis of 

 Spectrum Analysis, land had pointed out hotv it should 

 be applied to the detection of the constituents of the 

 atmospheres of the sun and stars. Since 1852 Thomson 

 has constantly given this as a part of his annual course of 

 Natural Philosophy in the University of Glasgow ; but, 

 till 1859, under the impression that it was quite well 

 known to scientific men. Balfour Stewart's experiments 

 and reasoning date from 1858 only, and those of Kirchhoflf 

 from 1859. 



In some of Stokes' earlier hydrokinetic papers, he for 

 the first time laid down the essential distinction between 

 rotational, and differentially irrotational, motion, which 

 forms the basis of Helmholtz's magnificent investigations 

 about vortex-motion. 



Another most valuable paper (a short abstract of which, 

 in the Reports of the British Association for 1857, seems 

 to be all that has been published) completely clears up the 

 difficulties which had been .felt with regard to the very 

 curious effects of wind upon sound, and the diffraction of 

 waves in air. The singular fact noticed by Sir John 

 Leslie that the intensity of a sound depends, ceteris 

 paribus, to a marked extent upon the nature of the gas in 

 which it is produced, is explained in an admirable 

 manner by Stokes in the Philosophical Transactions for 

 1868 in a paper entitled " On the Communication of 

 Vibration from a Vibrating Body to the surrounding 

 Gas." 



Of late years Stokes has^not published so many papers 

 as formerly : one reason at least has been already hinted 

 to the reader. But there is another. It is quite well 

 known that he has iri retentis several optical and other 

 papers of the very highest order, but cannot bear to bring 

 them out in an incomplete or hurried form. No doubt he 

 may occasionally hint at their contents in his lectures, 

 but his (undergraduate) audience are likely to take them 

 for well-known and recognised facts [as Thomson unfor- 

 tunately did in the case of Spectrum Analysis], and so 



