Interference Phenomena. 513 



information concerning a regularity " of vibration. But in 

 the first place we must ask ourselves whether we can attach 

 any meaning to the expression " regularity " of vibration 

 when applied to white light. 



5. To quote Lord Eayleigh once more: — " It would be in- 

 structive if some one of the contrary opinion would explain 

 what he means by regular white light. The phrase certainly 

 appears to me to be without meaning — what Clifford would 

 have called nonsense"*. 



I believe that those who speak of the regularity of vibration 

 in a continuous spectrum have in their mind the state of 

 motion of the original vibrating system, and not that of the 

 medium through which the light is transmitted. If we 

 imagine the light to be produced by a number of different 

 vibrating systems, each sending out homogeneous vibrations, 

 prismatic decomposition would give what we should call a 

 line spectrum. We may imagine the lines of the spectrum 

 to be so close together that any resolving power which we 

 can apply fails to separate them. 



There is nothing to prevent our imagining for the sake of 

 argument that all continuous spectra if examined by resolving 

 powers, say a million times larger than we can apply at 

 present, would ultimately appear to be line spectra, each line 

 representing perfectly homogeneous light ; and if each line 

 is due to a separate vibrating system, we may reasonably say 

 that that system remains perfectly regular for an indefinite 

 time. Let us then imagine a practically continuous spectrum 

 of this nature in which, for the sake of simplicity, there is no 

 difference in intensity, all wave-lengths being equally repre- 

 sented. The spectrum might be made completely continuous 

 by imagining each vibrating system to be split up into 

 several, moving with uniform velocities in different directions, 

 and that velocity may be made indefinitely small, as the 

 difference in wave-length between the vibrating systems is 

 indefinitely reduced. But for my purpose it will be sufficient 

 to take the spectrum as ultimately discontinuous, and I shall 

 take such a system of vibrations as the representative of what 

 may be called " regular " white light. On the other hand, 

 imagine a set of molecules setting up a luminous disturbance 

 by perfectly irregular and indefinitely short impulses. When 

 examined by a spectroscope we should see a continuous 

 spectrum ranging over all wave-lengths with equal inten- 

 sities ; of such we may imagine u irregular " white light 

 to consist. 



The question I shall discuss in detail is this : — Can we by 



* Phil. Mag. xxvii. p. 463, note (3889). 



