312 Sir J. J. Thomson on a Theory of 



be excited by resonance and will be in phase relation with 

 the primary vibration. Again, as the vibration from the 

 corpuscle travels through the luminous body it strikes 

 against other corpuscles, and excites in them vibrations 

 which are in definite phase relations with the primary vibra- 

 tions. It appears therefore that any vibration excited in a 

 single tube of force will, after it has travelled through the 

 parts of the luminous body surrounding the corpuscle at its 

 extremity, be accompanied by a number of secondary vibra- 

 tions in other tubes of force in phase relationship with the 

 primary vibration. So that even on the view that the 

 vibrations which constitute light travel along discrete lines 

 of force, the phases of the vibrations along the different 

 tubes of force which pass through any area under considera- 

 tion, such as the slit used to produce the diffraction -fringes, 

 will not all be independent. There will be groups of lines 

 of force in which the vibrations are connected by definite 

 phase relations, and the secondary waves started when the 

 vibrations of the different members of one of these groups 

 strike against the boundary of the slit, will also be in phase 

 relation, and will therefore be able to interfere. 



The existence of interference even when, as in experiments 

 such as those made by Mr. Taylor (Proceedings of the 

 Cambridge Phil. Soc. xv, p. 114), in which sharply defined 

 interference-fringes were obtained when the intensity of the 

 light was reduced so much that it required an exposure of 

 days or weeks to photograph the interference-fringes, is thus 

 not incompatible with the view that light consists of vibrations 

 travelling along discrete tubes of electric force. Experi- 

 ments with very feeble light do, however, show, unless we 

 assume that the amplitude of the transverse vibrations of 

 a tube of electric force can be greater than the width of the 

 slit, that the energy in the vibrations running along the 

 different tubes of force cannot all be equal unless we suppose 

 that this constant value is exceedingly small, much smaller 

 than the unit of radiant energy for light in the visible part 

 of the spectrum given by Planck's theory : for in Mr. Taylor's 

 experiments I estimate that the light was so feeble that 'there 

 was less than one of Planck's units per litre : unless the units 

 came very irregularly from the luminous body it is difficult 

 to imagine that they could be sufficiently crowded together 

 to interfere. 



On the effect of a magnetic field on the set of the tube 

 of electric force attached to a corpuscle. 

 We have seen that the axis of a tube of force attached to 

 a single corpuscle moving uniformly along a straight line, 



