496 Lord Kelvin on Continuity in Undulatory Theory of 



and rarefaction at every point of the equatorial plane and 

 maximum in the axial line. When the vibrating shell is 

 embedded in an elastic solid extending to vast distances in all 

 directions from it, two sets of waves, distortional and con- 

 densational-rarefactional, according respectively to the two 

 descriptions which have been before us, proceed outwards 

 with different velocities, that of the former essentially less 

 than that of the latter in all known elastic solids'*. Each 

 of these propagational velocities is certainly independent of 

 the frequency up to 10 4 , 10 5 , or 10 6 , and probably up to any 

 frequency not so high but that the wave-length is a large 

 multiple of the distance from molecule to molecule of the 

 solid. When we rise to frequencies of 4 x 10 12 , 400 x 10 12 , 

 800 xlO 12 , and 3000 x 10 12 , corresponding to the already 

 known range of long-period invisible radiant heat, of visible 

 light, and of ultra-violet light, what becomes of the conden- 

 sational -rare factional waves which we have been considering ? 

 How and about what range do we pass from the propaga- 

 tional velocities of 3 kilometres per second for distortional 

 waves in glass, or 5 kilometres per second for the con- 

 densational waves in glass, to the 200,000 kilometres per 

 second for light in glass, and, perhaps, no condensational 

 wave ? Of one thing we may be quite sure ; the transition 

 is continuous. Is it probable (if aether is absolutely incom- 

 pressible, it is certainly possible) that the condensational- 

 rarefactional wave becomes less and less with frequencies of 

 from 10 6 to 4 x 10 12 , and that there is absolutely none of it 

 for periodic disturbances of frequencies of from 4 x 10 12 to 

 3000 x 10 12 ? There is nothing unnatural or fruitlessly ideal 

 in our ideal shell, and in giving it so high a frequency as the 

 500 x 10 12 of yellow light. It is absolutely certain that there 

 is a definite dynamical theory for waves of light, to be 

 enriched, not abolished, by electromagnetic theory ; and it is 

 interesting to find one certain line of transition from our dis- 

 tortional waves in glass, or metal, or rock, to our still better 

 known waves of light. 



I. (2). Here is another still simpler transition from the dis- 

 tortional waves in an elastic solid to waves of light. Still 

 think of our massless rigid spherical shell, 13 centim. internal 

 diameter, with our solid globe of platinum, 12 centim. dia- 

 meter, hung in its interior. Instead of as formerly applying 

 simple forces to produce to-and-fro rectilinear vibrations of 

 shell and nucleus, apply now a proper mutual forcive between 

 shell and nucleus to give them oscillatory rotations in contrary 

 directions. If the shell is hung in air or water, we should 



* Math, and Phys. Papers, vol. iii. art. civ. p. 522. 



