54 



SCIENTIFIC THOUGHT. 



36. 



View of the 

 ether as an 

 " elastic 

 solid." 



The latest discussion of this form of the elastic-solid 

 theory of light, which was gradually developed from 

 independent beginnings in the three countries, 1 is to be 



1 In France and Germany, where 

 even in the middle of the century 

 the labours of English natural phil- 

 osophers like Green, M'Cullagh, 

 Stokes, were only very imperfectly 

 known, the necessity was equally 

 felt of studying the interaction of 

 the ether and ponderable matter. 

 In France the school of the eminent 

 "elastician," Barre" de St Venant, 

 produced in M. Boussinesq the author 

 of the earliest published attempt to 

 solve the difficulties which the older 

 methods of Cauchy had not over- 

 come. In a lucid review of the state 

 of physical optics, Saint Venant 

 himself ( ' Ann. de Chimie et de 

 Physique,' 4 me se"rie, vol. 25, 1872) 

 hails with delight the researches of 

 M. Boussinesq from 1865 onward, 

 where the idea that the ether in the 

 interstices of transparent bodies has 

 different elastic constants is given 

 up, and the participation of the 

 ponderable matter in the vibrations 

 is introduced in its place. "En 

 effet," he says, " il est bien difficile 

 de concevoir, d'uue part, que 1'dtber 

 puisse etre agile" au sein d un corps 

 dont la densite est probablement 

 bien supeYieure a la sienne, sans 

 lui communiquer une fraction sens- 

 ible de sa quantit^ de mouvement, 

 et d'autre part, que les ondes ne 

 soient pas bientot e"teintes par cette 

 participation de la matiere pondeY- 

 able au mouvement s'il n'y a pas 

 concordance entre les oscillations 

 imprimees a chaque molecule de 

 cette matiere et celles de Father 

 qui 1' environne." It was the 

 problem of the continuity at the 

 interface of reflecting and refracting 

 substances and the problem of ab- 

 sorption which the older simple 

 ether theories could not explain. 



In Germany a similar impulse was 

 given to the study of the inter- 

 action of elastic systems as indeed 

 to many problems of mathematical 

 physics by Franz Neumann, who 

 was the centre of a numerous and 

 influential school. He taught at 

 Konigsberg together with Richelot 

 and Bessel. His lectures have been 

 edited by his pupils. Prof. Karl 

 Pearson, in his continuation of Tod- 

 hunter's ' History of the Theory of 

 Elasticity,' does ample justice to the 

 labours of Neumann, who, "in his 

 investigations on photo - elasticity 

 and the elasticity of crystals, breaks 

 almost untrodden ground, which 

 both physicists and mathematicians 

 have hardly yet exhausted " (loc, cit. , 

 vol. ii. 2, p. 183). "Neumann was 

 among the first (1841, ' Abh. der Ber- 

 liner Akademie') to attribute disper- 

 sion to the influence of the ponder- 

 able particles on the particles of the 

 ether" (ibid., p. 31). The most 

 important original contributions of 

 Neumann's pupils are the researches 

 of Sellmeier, who had been led by 

 theoretical considerations in 1866 

 to expect certain anomalies in the 

 phenomena of dispersion, such as 

 were in 1870 actually discovered by 

 Christiansen, and fully investigated 

 by Kundt. Surface coloration was 

 shown to be intimately connected 

 with the absorptive powers in sub- 

 stances showing these anomalous 

 phenomena. A full report on these 

 and other theories, based upon what 

 has been termed abroad the " Bessel- 

 Sellmeier hypothesis " (see Ket- 

 teler, ' Theoretische Optik,' 1885), 

 will be found in Prof. Glazebrook's 

 " Report on Optical Theories," 

 Brit. Assoc. Reports, 1885. 



