Photographic Action of Stationary Light-Waves. 97 



would appear to indicate. This will be evident from a con- 

 sideration of the method used on page 241 of the ' Physics 

 of the Earth's Crust,' and its subsequent applications in 

 chapters xvii. and xxvi. We have, in each of the cases dis- 

 cussed, gone to the furthest approximation allowable under 

 the circumstances. The limitations are made perfectly clear 

 in the book, and I do not think that there is anything to be 

 added to this part of the work. 



Finally, we see that we are left with a residual effect, which 

 is undoubtedly very small, and it is not impossible that such 

 an effect might exist in the case of the Earth. 



V. The Significance of Wiener s Localization of the Photo- 

 graphic Action of Stationary Light- Waves. By J . Larmor, 

 F.R.S., Fellow of St. Johns College, Cambridge* '. 



THE experiments by which Wiener demonstrated f that, 

 when stationary plane-polarized optical undulations are 

 produced in a photographic film, by reflexion of a stream of 

 incident plane-polarized light at a metallic or other backing, 

 the photographic action occurs at the antinodes of Fresnefs 

 vibration-vector and not at the nodes, have been employed by 

 its author and others to decide between the various theories 

 of light. If for purposes of precise description we utilize 

 the terminology of the electric theory of light, which formally 

 includes all the other theories by proper choice of the vibra- 

 tion-vector, w r e may say that the photographic action takes 

 place at the antinodes of the electric vector which corresponds 

 to Fresnel's vibration, and not at the intermediate antinodes 

 of the magnetic vector ivhich corresponds to MacCullagh's 

 and to Neu mannas vibration. 



The crucial experiment of Wiener relates to the case when 

 the angle of incidence is half a right angle, so that the direct 

 and reflected waves which interfere are at right angles to each 

 other. If the vibration take place along the direction of 

 intersection of the two wave-planes, it will present a series of 

 nodes and antinodes ; but if in the perpendicular direction 

 there will not be such alternations of intensity. The experi- 

 ment showed that when the light is polarized in the plane of 

 incidence, the photographic plate develops a series of bands ; 

 but when it is polarized in the perpendicular plane these 

 bands are absent. 



The argument employed is that the photographic effect will 



* Conimimicated by the Physical Society: read November 9, 1894. 

 t Wiedemann's Annalen, 1890. 



Phil. Mag. S. 5. Vol. 39. No. 23u. Jan. 1895. H 



