782 REPORT— 1898. 



2. On a Magnifying Telephone.^ By Professor Oliver Lodge, F.R.S. 



For tlie purpose of ' calling- up ' in a system of magnetic induction telegraphy, and 

 for other purposes, the author has devised a kind of telephone which emits a loud 

 sound when stimulated by an exceedingly feeble alternating current. This is done 

 partly by dispensing with iron in the moving: part and utilising the motion of the 

 coil instead, partly by aid of a large wooden sound-board, and partly by a com- 

 bination of telephones and microphones in series, alternately electrically and 

 mechanically connected, each microphone receiving a disturbance mechanically 

 and passing it on electrically in a magnified condition to the next by reason of the 

 fresh energy of its battery. 



3. On the Measurement of Small Differences in Resistance. 

 By E. H. Griffiths, F.R.S. 



4. The Dynamical Theory of Refraction, Dispersion, and Anomalous 

 Dispersion. By Lord Kelvix, G.C.V.O. 



The dynamical theory of dispersion, as originally given by Sellmeier,'' con- 

 sisted in finding the velocity of light as affected by vibratory molecules embedded 

 in ether, such as those which had been suggested by Stokes ^ to account for the 

 dark lines of the solar spectrum Sellmeier's mathematical work was founded on 

 the simplest ideal of a molecular vibrator, which may be taken as a single material 

 particle connected by a massless spring or springs with a rigid lining of a small 

 vesicle in ether. He investigated the propagation of distortional waves, and 

 found the following expression (which I give with slightly altered notation) for 

 the square of the refractive index of light passing through ether studded with a 

 very large number of vibratory molecules in every volume equal to the cube of 

 the wave-length 



2 2 3 



;x' = 1 + m^!— + m, ^^ + w,, ~ — - + &c., 



where r denotes the period of the light ; k, k^, k,,, &c., the vibratory periods of the 

 embedded molecules on the supposition of their sheaths held fixed; and m, m^, m^^, 

 &c., their masses. He showed that this formula agreed with all that was known 

 in 1872 regarding ordinary dispersion, and that it contained what we cannot 

 doubt is substantially the true dynamical explanation of anomalous dispersions, 

 which had been discovered by Fox-Talbot ■* for the extraordinary ray in crystals 

 of a chromium salt, by Leroux '' for iodine vapour, and by Christiansen" for liquid 

 solution of fuchsin, and had been experimentally investigated with great power 

 by Kuudt." 



Sellmeier himself somewhat marred ^ the physical value of his mathematical 

 work by suggesting a distinction between refractive and absorptive molecules 

 (' refractive and absorptive Theilchen '), and by seeming to confine the application 

 of his formula to cases in which the longest of the molecular periods is small in 

 comparison with the period of the light. But the splendid value of his formula 

 for physical science has been quite wonderfully proved by Rubens, who, however, 



' See Proc. Inst. Elec. Engineers, Dec. 1898. 



2 Sellmeier, Pogg. Ann., vol. cxlv. 1872, pp. 399, 520 ; vol. cxlvii. 1872, pp. 38G 



525 



3 



See KirchhofE-Stokes-Thomson, Phil. Mag., March and July 1860. 

 ♦ I'.'x-Talbot, Proc. Roy. Soc. Edin., 1870-71. 

 5 ! eroux, Comjdcs Hindus, vol. Iv. 1862, pp. 126-128. 



" Christiansen, Pogg. Ann., vol. cxli. 1870, pp. 479, 480 ; Phil. Mag., vol. xli. 1871 

 244 ; Annales de Chimic, vol. xxv. 1872, pp. 213, 214. 

 ' Kundt. Pogg. Ann., vob. cxlii., cxliii., cxliv., cxlv. 1871-72. 

 ' Poig. Ann., vol. cxlvii. 1872, p. 525. 



