Condensatlonal-raref actional Waves in Gases Sfc. 499 



and for our massive platinum nucleus a massless non-con- 

 ducting globe electrified with a given quantity of electricity. 

 For simplicity we shall suppose our apparatus to be surrounded 

 by air or aether. 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-conductor coated with 

 two circular caps of metal, leaving an equatorial non- 

 conducting zone between them, and let thin wires from a dis- 

 tant alternate-current dynamo, or electrostatic inductor, give 

 periodically varying opposite electrifications to the two caps. 

 For moderate frequencies of vibration we have a periodic 

 variation of electrostatic force in the air or aether surrounding 

 the apparatus, which we can readily follow in imagination, 

 and can measure by proper electrostatic measuring apparatus. 

 Its phase, with moderate frequencies of vibration, is very 

 exactly the same as that of the electric vibrator. Now sup- 

 pose the frequency of the vibrator to be raised to several 

 hundred million million periods per second. We shall have 

 polarized 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 phase of variation of the 

 electrostatic force in the axial line outside the apparatus 

 remain exactly the same as that of the vibrator? An affirma- 

 tive answer to this question would mean that the velocity of 

 propagation of electrostatic force is infinite. A negative 

 answer would mean that there is a finite velocity of propaga- 

 tion for electrostatic force. This velocity, according to views 

 regarding conceivable qualities of aether described in my 

 article " On the Reflection and Refraction of Light " (Phil. 

 Mag. vol. xxvi. 1888) might be greater than, equal to, or less 

 than the velocity of light. 



III. The shell and interior electrified non-conducting mass- 

 less globe being the same as in II., let now a forcive be 

 applied between shell and nucleus to produce rotational oscil- 

 lations as in I. (2). When the frequency of the oscillations 

 is moderate, there will be no alteration of the electrostatic 

 force and no perceptible magnetic force in the air or aether 

 around our apparatus. Let now the frequency be raised to 

 several hundred million million periods per second ; we shall 

 have visible polarized light proceeding as if from an ideal point- 

 source at the centre and answering fully to the description of 

 the light of I. (2). The same result would be obtained by 

 taking simply a fixed solid non-conducting globe and laying 

 on wire on its surface approximately along the circumferences 

 of equidistant circles of latitude, and, by the use of a distant 

 source (as in II.), sending an alternate current through this 



