08 Mr. J. Larmor on Wiener s Localization of the 



be greatest at those places in the stationary wave- train where 

 the vibration is most intense ; and the conclusion is drawn 

 from it that the actual vibration is represented by Fresnel's 

 vector and not by MacCulIagb/s ; in other words, that the 

 vibrations of polarized light are at right angles to the plane 

 of polarization. The force of this argument, as against 

 MacCullagh's theory, would, however, be evaded if the vector 

 of that theory were taken to represent something different 

 from the linear displacement of the aether, or if vibrations 

 were excited in the molecule by rotation instead of transla- 

 tion, or by stress, as Poincare has pointed out *. 



But as a matter of fact it seems difficult to assign any 

 reason of the above simple kind, on either theory, in favour 

 of the photographic disturbance occurring at the antinodes 

 rather than at the nodes of the optical vibration. The re- 

 markable suggestion thrown out by Lord Eayleigh some time 

 previous to Wiener's experiments, and afterwards verified by 

 Lippmann, that certain effects in colour photography produced 

 by Fox Talbot and Becquerel were really due to this kind of 

 localization of the photographic effect, is not in opposition to 

 such a view ; for the consideration adduced was simply that 

 a localization, periodic with the waves, would, if it happened 

 to exist, produce effects like the observed ones. At any rate, 

 the observed localization demonstrates the important result 

 that the effect is due to a specific dynamical action of the 

 waves, and not to mere general absorption of the radiation. 



Let us consider the actual circumstances of the case. 

 There are about 10 3 molecules of the sensitive medium in the 

 length of a single wave of light : thus in the stationary wave- 

 train all the parts of a single molecule would at any instant 

 be moving with a sensibly uniform velocity, which increases 

 and diminishes periodically, The vibration of the molecule 

 would thus be, were it not for the influence of differences of 

 inertia or elasticity between its parts and the surrounding 

 aether, very nearly a swaying to and fro of it as a whole : 

 if it were exactly this, it could not be expected to produce 

 any breaking up of the molecule at all. Moreover, as at the 

 antinodes of the vibration there is movement but no stress in 

 the medium, so at the nodes there is stress but no movement; 

 and it does not seem at all clear that alternating stress might 

 not be as potent a factor in disintegration as alternating- 

 motion. A representation has been constructed by Lord 

 Kelvin f of a system in which internal vibrations can be 



* See a discussion on this subject in Comptes Rendus, cxii. 189], in 

 which MM. Cornu, Poincare, and Potier took part. 



t " Lectures on Molecular Dynamics," Baltimore, 1884. 



