On Approximately Simple Waves. 135 



tifieation detached itself from the anode and joined the stra- 

 tifications in the narrow portion of the tube. This process 

 could be repeated until there appeared to be formed a species 

 of stationary wave, due apparently to the setting free of the 

 gases from the anode meeting a cathode liberation. When 

 the strength of the current is diminished, the stratifications 

 disappear in the terminals, more noticeably in the anode than 

 in the cathode. This process can be traced to the a?-ray 

 stage. Since there are no electrical oscillations in the circuit, 

 for I had several megohms in circuit, the molecular theory 

 of bombardment, together with the phenomenon of occlusion, 

 seem to be most important. The end-on tube which was 

 employed was filled with dry hydrogen obtained by electrolysis 

 from phosphoric acid and distilled water. The gas was passed 

 through caustic potash and phosphoric pentoxide. Hittorf's 

 resistance of iodide of cadmium in amyl alcohol will un- 

 doubtedly be better than distilled water in quantitative 

 experiments. 



Jefferson Physical Laboratory, 

 Cambridge, U.S. 



VII. On Approximately Simple Waves. 

 By Lord Rayleigh, F.R.S.* 



THE first question that arises is as to the character of 

 absolutely simple waves ; and here " it may be well to 

 emphasize that a simple vibration implies infinite continuance, 

 and does not admit of variations of phase or amplitude. 

 To suppose, as is sometimes done in optical speculations, 

 that a train of simple waves may begin at a given epoch, 

 continue for a certain time involving it may be a large 

 number of periods, and ultimately cease, is a contradiction 

 in terms" f. A like contradiction is involved if we speak of 

 unpolarized light as homogeneous, really homogeneous light 

 being necessarily polarized. 



This much being understood, approximately simple waves 

 might be defined as waves which for a considerable succession 

 deviate but little from a simple train. Under this definition 

 large changes of amplitude and frequency would not be 

 excluded, provided only that they entered slowly enough. 

 More frequently further limitation would be imposed, and 

 approximately simple waves would be understood to mean 

 waves which for a considerable succession can be approximately 

 identified with a simple train of given frequency, if not of 



* Communicated by the Author. 



t 'Theory of Sound,' 2nd ed. § 65 a, 1891. 



