KINETIC OR MFX'HANICAL VIEW OF NATIKK. 41 



France by Na\'ier and I'oisson. One of the greatest 

 analysts of the century, Augustin L'auchy, had like- 

 wise applied himself to it; and when Fi-esnel, in ihe 

 \ear 1S26, brought out his great memoir on double 

 refraction in crystals, in which he was obliged to enter 

 more closely into the properties of the luminiferous ether 

 and its relation to ponderable matter, Cauchy was induced 

 to devote himself more specially to the mathematical 

 problems which presented themselves. Before his time 

 the theory of elasticity had been studied more as con- 

 nected with questions of practical engineering, such as 

 the strength of materials, the stability of buildings, the 

 construction of macliines, or with the properties of nnisical 

 and sounding bodies. A new interest was created by 

 Fresnel's researches.^ The ([uestion arose, \hi\\ mv we 

 to describe the vibrations of an imponderable substance, 

 endowed with mass (density) and rigidity, and what con- 

 ceptions can we form of the change of these vibrations 

 if there is present likewise ponderable matter ? Evi- 

 dently upon the clearness and correctness of these 

 notions depends the explanation of the phenomena observ- 

 able when rays of light fall upon the surfaces of trans- 

 parent or opaque bodies. We have to ask: In wliat 

 terms (viz., of different kinds of motion) can we define 

 and describe, and accordingly calculate the phenomena 

 of reflexion, refraction, scattering (i.e., dispersion), and 

 absorption (i.r., extinction) of light ? A tolerably clear 



1 See Verdet iu ' ffiuvres de 

 Fresnel,' vol. i. p. Ixxx : " Les 

 seuls ecrits antdrieurs a Fresnel 

 oil I'on trouve des notions justes 

 8ur les in(5galites d't'^liisticito qui 

 peuvent exister dans les corps et 



sur leur repartition rt^Kuli6re ]mr 

 rapport h, certains axes ou plans de 

 symdtrie sont ceux du grand miiu'"- 

 ralogiste allemand Samuel Christian 

 Weis" (' Mdm. de I'AwkI. dc Herlin.' 

 ISl.'i). 



