the Structure of the filed vie Field. 311 



tubes which are transmitting the energy are those which are 

 terminated by the moving charged particles which are the 

 source of the light. The energy would thus be localized 

 along these tubes, and would not be distributed continuously 

 throughout the space within sight of the luminous body. 

 The theory would have some of the characteristics of both 

 the emission and the usual form of the undulatory theory. 

 It would agree with the emission theory in supposing that 

 the energy of light is concentrated in small specks in the 

 space round the luminous body, while it would be in agree- 

 ment with the undulatory theory in supposing that the 

 disturbance whose propagation constitutes light is a vector 

 quantity. I sketched a theory of this kind in my ' Electricity 

 and Matter,' and pointed out there that the electrical pro- 

 perties of light, such as the ionization of gases by ultra-violet 

 light and photo-electric effects, could be more easily explained 

 by a theory of this kind than by one requiring a continuous 

 distribution of energy throughout the aether. 



With respect to purely optical effects, the explanation of 

 most of these would be the same on either theory ; the case of 

 interference, however, requires special consideration. Inter- 

 ference implies that over the slit, or obstacle, or whatever 

 system is used to produce the interference, the disturbance 

 producing the light should be in the same phase, or at any 

 rate that the disturbances at different parts should have their 

 phases definitely related. It is not necessary that this 

 disturbance should be absolutely continuous over the slit, if, 

 for example, the disturbances travelling along a number of 

 discrete lines of force passing through the slit reached the 

 slit in the same phase, we should get much the same set of 

 fringes as if the disturbance had been uniformly distributed 

 over the slit. If, however, there were no phase relations 

 between the vibrations of the tubes of force as the disturbance 

 passed through the slit, there would not be interference 

 unless the amplitude of the vibration extended from one side 

 of the slit to the other, so that the electric force in the 

 disturbance would excite secondary vibrations in the cor- 

 puscles in the metal on the two sides of the slit : as these 

 secondary vibrations would be in the same phase they would 

 be able to produce interference effects. In any realizable 

 case it would seem, however, that the vibrations along a 

 considerable number of tubes coming from a luminous body 

 must have their phases related in a definite way. For 

 consider a corpuscle vibrating in a definite period ; in its 

 neighbourhood there will be many other systems having 

 the same time of vibration, and the vibrations of these will 



