324 Profs. H. du Bois and H. Rubens on Polarization of 



thickness of the wire and apparent slit- width ; n 2 seemed to 

 depend only on the latter, at least within the region investi- 

 gated. This simple course was in accordance with expecta- 

 tions, for in the infra-red, apart from the more favourable 

 ratio b'/X, the molecular periods of the metals play only a 

 small part, while in the visible part of the spectrum they 

 completely dominate their behaviour. 



After some experiments on partial polarization by means 

 of ruled gratings had been described in the first publication 

 on Reststrahlen*, we applied a platinum- wire grating to the 

 Reststrahlen of fluorite and rocksaltf. These were polarized 

 by reflexion from glass and quartz plates at the polarizing 

 angle. It was found also in the long-wave region that a 

 decrease in the ratio of transmissibility took place with 

 increasing wave-length, thus showing a closer approach to 

 complete Hertzian polarization. 



§3. Sir J. J. Thomson | has calculated the polarization 

 through a grating the interspaces of which are small in 

 comparison with the wave-length, as well as the scattering 

 of light by a metallic or dielectric cylinder. 



Lord Rayleigh § has solved the mathematical problem of 

 the transmission of wave disturbances through openings in 

 plane thin infinite conducting sheets. He found that a 

 narrow slit — of width infinitely small in proportion to the 

 wave-length — parallel to the electric vibration transmits a 

 very much smaller quantity than the amount reflected by a 

 conductor which fits exactly in the same slit. He also 

 investigated the influence of ellipsoidal and elliptical cylin- 

 drical obstacles of dimensions again infinitely small in com- 

 parison with the wave-length, — a problem, the solution of 

 which we had already pointed out as desirable (/. c. § 33). 



With regard to such calculations Prof. Lamb || inves- 

 tigated the reflexion and transmission of electric waves 

 through metallic gratings, again under the assumption that 

 the distance of the strips from one another was small in 

 comparison with the wave-length ; otherwise no special 

 conditions were postulated. 



* H. Rubens and E. F. Nichols, Wied. Ann. lx. p. 418 (1897). 



t H. du Bois and H. Rubens, Verh. d. Deutsch. Phys. Get. vi. p. 77 

 1904) ; in the table on p. 81 the value of Q p for 25*5 /x should be 0-365 

 not 0*265, due to a printers' error). 



% J.J. Thomson, Rec. Res. in Electr. & Magn. §§ 359-388, p. 425 seq. 

 Oxford, 1893. The corresponding- calculations for conducting and 

 dielectric spheres, of late investigated by various authors, are only 

 indirect^ connected with the grating effect. 



§ Lord Rayleigh, Phil. Mag. [5] xliii. p. 259, xliv. p. 28 (1897). 



|| H. Lamb, Proc. Math. Soc. London, xxix. p. 523 (1893) j Beiblatter, 

 xxiii. p. 277 (1899). 



