Mr Campbell, Discontinuities in Light Emission. 311 



These theories were proposed by J. J. Thomson* and by Planck •}-. 

 They both suggest (at least if we interpret Planck's theory in the 

 sense given to it by Stark :]:) that the light from a single radiator 

 is not emitted equally in all directions, but is concentrated along 

 a finite number of narrow tubes radiating from the radiator : they 

 also suggest that the energy emitted by such a radiator in virtue 

 of a single disturbance is not infinitely divisible, but only divisible 

 into a finite number of equal parcels, the magnitude of which, 

 according to Planck, depends on the frequency : further, this finite 

 number is usually quite small under experimental conditions. 



This view is capable of explaining many difficulties connected 

 with the ionisation of gases and allied phenomena which are 

 quite incomprehensible on the older theory that the light spreads 

 out from the radiator as a spherical wave. But it also raises some 

 new difficulties in connection with optical interference. If the 

 light from a single radiator can only be split into a small number 

 of parts, then, if the beam of light from an ordinary source, con- 

 taining a great number of separate radiators, is divided by any of 

 the ordinary interference methods, the light in one beam must, in 

 general, come from radiators different from those which emit the 

 light in the other. Two rays of light which come from different 

 radiators will be termed ' independent.' 



And hei'e a distinction, which may be of importance for oar 

 purpose, must be made between the two methods of exciting inter- 

 ference. In the first method, of which Fresnel's mirrors are 

 typical, the two beams which are made to interfere eventually 

 are emitted at different angles from the source: they will be 

 independent, in our present sense, if it be true that the light 

 from a single radiator is emitted in one direction only, and 

 this conclusion requires no assumptions as to the divisibility of 

 the energy in the light from a single radiator. In the second 

 method, exemplified by Michelson's interferometer, the two beams 

 proceed from the source in the same direction. They will be 

 independent, only if it is impossible to split up the energy from a 

 single radiator into two parts, so that the beam reflected by the 

 half-silvered mirror must come from different radiators from that 

 transmitted by it. On Planck's theory of radiation there should 

 be no distinction important for our purpose between the two 

 interference methods, but on Thomson's theory it is possible that 

 the two beams produced by the second method are dependent, 

 while those produced by the first are independent. 



Now it is not difficult to imagine why two independent beams 



* See Electricity and Matter, London, 1904, p. 63, or Proc. Camb. Phil. Soc. xiv. 

 1907, p. 417. 



t M. Planck, Wdrmestrahlung , Leipzig, 1906. 

 J J. Stark, Phijs. Zeit. x. 1909, p. 579. 



