498 Lord Kelvin on Continuity in Undulatory Theory of 



if, as in all probability is the case, the distance between the 

 centres of contiguous molecules in glass and in iron is less 

 than a five-hundredth of a mikrom. But it is probable that 

 the distance between centres of contiguous molecules in glass 

 and in iron is greater than 10 -5 of a mikrom, and therefore it 

 is probable that neither of these solids can transmit waves of 

 distortional motion of their own ponderable matter, of so short 

 a wave-length as I0 -5 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 6 , that is 

 to say, to 100 x 10 12 , no distortional wave of motion of the 

 ponderable matter can be transmitted outwards; but it seems 

 quite certain that distortional waves of radiant heat in aether 

 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, thickness and of refrac- 

 tive index 1*5 for D light. Let the frequency of the oscil- 

 lations be increased to 5*092 x 10 14 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 § x '589212 of a mikrom, and out 

 from the glass into air with wnve-lengih 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 polarization will be everywhere the meridional 

 plane. It is interesting to remark that the axis of rotation of 

 the aether for this case coincides everywhere with the line of 

 vibration of the aether 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. 



4l 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 sub- 

 stituting for our massless shell an ideally rigid, infinitely 

 massive shell of glass or other non-conductor of electricity, 



