470 Proceedings of the Koyal Society of Edinburgh. [Sess. 
following passage, quoted from Professor Schuster’s article “ On Interfer- 
ence Phenomena ” referred to above: — “We cannot help speculating as to 
the ultimate cause which renders the regularity of vibration a function of 
the temperature only, and independent of the natural periods of the 
molecules. Perhaps the solution of the difficulty will be found in the fact 
that our observations tell us nothing directly as to the vibrations of the 
atoms or molecules. What we observe is the disturbance of the medium ; 
and the distribution of energy in the spectrum of an incandescent black 
body which is in thermal equilibrium may indicate a property of the 
medium rather than that of matter. That is t6 say, the motion of vibration 
in the molecule may be perfectly irregular, but the medium may take up 
and propagate some vibrations quicker than others.” 
§ 29. In a later paper I hope to illustrate further the process of dis- 
persion by showing the wave - systems arising from the same initial 
disturbance in several media having different dispersive qualities. 
( Issued separately July 16, 1909.) 
