355. ABERRATION IN A DISPERSIVE MEDIUM. [Plnloxophmd Magazine, Vol. xxn. pp. 130—134, 1911.] THE application of thu theory of group- velocity to the; case of light was in an early paper* in connexion, with Home experimental results announced by Young and Forbonf. It IB now, I believe, generally agreed that, whether the, method be that of the toothed wheel or of the revolving mirror, whali Is determined by the experiment in not V, the wavo-velocity, bud U, the group-velocity, where k being invernely HH the wave-length. In a dispersive medium Fund tTare different. I proeeede,d:—"Thu evidence of the terrestrial methods relating exclusively to IJ\ we turn to conHi'der the astronomical methods. Of these there are two, depending respectively upon aberration find upon the eclipses of Jupiter'H HatelliUu The labtx-r evidently gives U. The former dooH not depend upon obnei'ving the propagation of a pt^culiarity impressed upon a train of wavcn, and therefore, haw no relation to U, If we accept the usual theory of aberration HH HatiHfactory, the roHiilt of a compariHon between the1 coefficient found by obnervation arid the solar parallax is F— the wave-velocity," The above abortion that Htellar aberration given V rather than 6r has recently been callod in queation by Khrenfeatj', and with good reawon. He nhoWH that tha circumsfcancoH do not < lifter materially from those of the toothed wheel in Fistcau's method. The argument that ho employs bears, indeed, close affinity with the method used by me in a later paper §. "The * Nature, Vols. xxiv., sxv, 1881 ; ttci&ntijlc Papers, Vol. I. p. 687. + Thew observers concluded that blue light travels in vacua 1-8 per cent, faster than red light. t Ann. d. /%»«, Bd. xxxni. p. 1571 (1910). | Xntitre, Vol. aav. p. 499 (1892); Scientific Papers, Vol. in. p. 542.h prevent the motion from maintaining its simp] stratified character. The resistance is then doubtless more nearly as tl: square of the velocity and independent of the value of v.