TRANSACTIONS OF SECTION A, 783 



inadvertently quotes * it as if due to Ketteler. Fourteen years ago Langley ^ had 

 measured the refractivity of rock-salt for light and radiant heat of wave-lengths 

 (in air or ether) from -43 of a mikron to 5-3 mikrons (the mikron heing 10"'' of a 

 metre, or 10~^ of a centimetre), and without measuring refractivities further had 

 measured wave-lengths as great as 15 mikrons in radiant heat. Within the last 

 six years measurements of refractivity by Rubens, Paschen, and others, agreeing 

 in a practically perfect way with Langley 's through his range, have given us very 

 accurate knowledge of the refractivity of rock-salt and of sylvin (chloride of 

 potassium) through the enormous range of from -4 of a mikron to 23 mikrons. 



Rubens began by using empirical and partly theoretical formulas which had 

 been suggested by various theoretical and experimental writers, and obtained 

 fairly accurate representations of the refractivities of flint-glass, quartz, fluor-spar, 

 sylvin, and rock-salt through ranges of wave-lengths from 'i to nearly 12 mikrons.* 

 Two years later further experiments extending the measure of refractivities of 

 sylvin and rock-salt for light of wave-lengths up to 23 mikrons showed deviations 

 from the best of the previous empirical formulas, increasing largely with increasing- 

 wave-lengths. Rubens then fell back '' on the simple unmodified Sellmeier formula, 

 and found by it a practically perfect expression of the refractivities of those 

 substances from '434 to 22'3 mikrons. 



And now for the splendid and really wonderful confirmation of the dynamical 

 theory. One year later a paper by Rubens and Aschkinass '" describes experiments 

 proving that radiant heat after five successive reflections from approximately parallel 

 surfaces of rock-salt and again of sylvin is of mean wave-length 51'2 and 61 •! 

 mikrons respectively. The formula which Rubens had given in February 1897, as 

 deduced solely from refractivities measured for wave-lengths of less than 23 mikrons, 

 made /u^ negative for radiant heat of wave-lengths from 37 to 55 mikrons in the 

 case of reflection from rock-salt, and of wave-lengths from 45 to 67 mikrons in the 

 case of reflection from sylvin ! {fj.^ negative means that waves incident on the 

 substance cannot enter it, but are totally reflected). 



5. Continuity in Undulatory Theory of C ondensational-raref actional 

 Waves in Gases, Liquids, and Solids, of Distortional Waves in Solids, 

 of Electric Waves in all Substances capable of transmitting them, and of 

 Badiant Heat, Visible Light, Ultra- Violet Light. By Lord Kelvin, 

 G.C. V.O. 



Consider the following three analogous cases : — I. mechanical, II. electrical, 

 III. electromagnetic. 



I. Imagine an ideally rigid globe of solid platinum of 12 centimetres diameter, 

 hung inside an ideal rigid massless spherical shell of 13 centimetres internal dia- 

 meter, and of any convenient thickness. Let this shell be hung in air or under 

 water by a very long cord, or let it be imbedded in a great block of glass, or rock, 

 or other elastic solid, electrically conductive or non-conductive, transparent or non- 

 transparent for light. 



I. (1) By proper application of force between the shell and the nucleus cause the 

 shell and nucleus to vibrate in opposite directions with simple harmonic motion 

 through a relative total range of 10~^ of a centimetre. We shall first suppose the 

 shell to be in air. In this case, because of the small density of air compared with 

 that of platinum, the relative total range will be practically that of the shell, and 



' Wicd. Ann., vol. liii. 1894, p. 267. In the formula quoted by Eubens from 

 Ketteler, substitute for juoo the value of /i found by putting t = oo in Sellmeier's 

 formula, and Ketteler's formula becomes identical with Sellmeier's. Eemark that 

 Ketteler's ' M ' is Sellmeier's 'wk'^,' according to my notation in the text. 



' Langley, PMl. Mag., 1886, second half-year. 



' Rubens, Wicd. Ahji., vols. liii. liv. 1894-95. 



* Rubens and Nichols, IPicrf. Ann. vol. Ix. 1896-97, p. 454. 



* Rubens and Aschkinass, Wied. Ann., vol. Ixiv. 1898. 



