786 REPORT— 1898. 



probable that neither of these solids can transmit waves of distortional motion of 

 their own ponderable matter of so short a wave-length as 10~' of a mikrom. 

 Hence it is probable that if we increase the frequency of the rotational vibrations 

 of our shell to one hundred thousand times 1000 x 10'', that is to say, to 100 x 10'-, 

 no distortional wave of motion of the ponderable matter can be transmitted out- 

 wards ; but it seems quite certain that distortional waves of radiant heat in ether 

 will be produced close to the boundary of the vibrating shell, although it is also 

 probable that if the surrounding solid is either glass or iron these waves will not 

 be transmitted far outwards, but will be absorbed, that is to say, converted into 

 non-undulatory thermal motions, within a few mikroms of their origin. 



Lastly, suppose the elastic solid around our oscillating shell to be a concentric 

 spherical shell of homogeneous glass of a few centimetres, or a few metres, thick- 

 ness and of refractive index 1-5 for D light. Let the frequency of the oscillations 

 be increased to 5092 x 10" periods per .second, or its period reduced to -589212 of a 

 michron : homogeneous yellow light of period equal to the mean of the periods 

 of the two sodium lines will be propagated outwards through the glass with 

 wave-length of about f x -589212 of a mikrom, and out from the glass into 

 air with wave-length of '589212 of a mikrom The light will be of maximum 

 intensity in the equatorial plane and zero in either direction along the axis, and its 

 plane of polarisation will be everywhere the meridional plane. It is interesting to 

 remark that the axis of rotation of the ether for this case coincides everywhere 

 with the line of vibration of the ether in the case first considered ; that is to say, 

 in the case in which the shell vibrated to and fro in a straight line, instead of, as 

 in the second case, rotating through an infinitesimal angle round the same line. 



A full mathematical investigation of the motion of the elastic medium at all 

 distances from the originating shell, for each of the cases of I. (1) and I. (2), will 

 be found in a volume containing my Baltimore Lectures on ' Molecular Dynamics 

 and the Wave- Theory of Light,' soon, I hope, to be published. 



II. An electrical analogy for I. (1) is presented by substituting for our mass- 

 less shell an ideally rigid, infinitely massive shell of glass or other non-conductor 

 of electricitj' and for our massive platinum nucleus a massless non-conducting globe 

 electrified with a given quantity of electricity. For simplicity we shall suppose our 

 apparatus to be surrounded by air or ether. Vibrations to and fro in a straight line 

 are to be maintained by force between shell and nucleus as in I. (1). Or, consider 

 simply a fixed solid non-conducting globe coated with two circular caps of metal, 

 leaving an equatorial non-conducting zone between them, and let thin wires from 

 a distant alternate-current dynamo, or electrostatic inductor, give periodically 

 vai'ying opposite electrifications to the two caps. For moderate frequencies we 

 have a periodic variation of electrostatic force in the air or ether surrounding the 

 apparatus, which we can readily follow in imagination, and can measure by proper 

 electrostatic measuring apparatus. Its phase, with moderate frequencies, is very 

 exactly the same as that of the electric vibrator. Now suppose the frequency of 

 tlie vibrator to be raised to several hundred million million periods per second. 

 We shall have polarised light proceeding as if from an ideal point-source at the 

 centre of the vibrator and answering fully to the description of I. (1). Does the 

 ])hase of variation of the electrostatic force in the axial line outside the apparatus 

 remain exactly the same as that of the vibrator P An affirmative answer to this 

 f(uestion would mean that the velocity of propagation of electrostatic force is 

 iufinite. A negative answer would mean that there is a finite velocity of propa- 

 gation for electrostatic force. This velocity, according to views regarding con- 

 ceivable qualities of ether described in my article ' On the Reflection and 

 liefraction of Light,' ' might be greater than, equal to, or less than the velocity of 

 light. 



III. The shell and interior electrified non-conducting massless globe being the 

 same as in II., let now a forcive be applied between shell and nucleus to produce 

 rotational oscillations as in I. (2). When the frequency of the oscillations is mode- 

 rate, there will be no alteration of the electrostatic force and no perceptible mag- 



« PJiil. Ma J., vol. XX vi. 1888. 



