MATTER AND ETHER THOMSON. 241 



of light as usually given, it is tacitly assumed that the electric force 

 is everywhere uniform over the wave front, that there are no vacant 

 spaces, and that the front has no structure. This is no necessary 

 part of the electric theory, and I think there is evidence that the 

 wave front does in reality much more closely resemble a number of 

 bright specks on a dark ground than a uniformly illuminated area. 

 Let me mention one such piece of evidence. If a flash of light, espe- 

 cially ultra-violet light, fall on a metal surface, negatively electrified 

 corpuscles are emitted from the surface; but when we measure, as 

 we can do, the number of these, we find that only a most insignifi- 

 cant fraction of the number of molecules passed over by the wave 

 front have emitted these corpuscles. If the wave front were continu- 

 ous, then all the molecules of the metal exposed to the light would 

 be under the same condition, and although, like the molecules of a 

 gas, the molecules might possess very different amounts of kinetic 

 energy, this difference would be nothing like sufficient to account 

 for the enormous discrepancy between the number of molecules struck 

 by the light and those which emit corpuscles. This discrepancy 

 would, however, easily be understood if we suppose that the wave 

 front is not continuous but full of holes, so that only a small number 

 of molecules come under the influence of the electric force in the 

 light. We may suppose that light consists of small transverse pulses 

 and waves traveling along discrete lines of electric force, dissemi- 

 nated throughout the ether, and that the diminution in the intensity 

 of the light as it travels outward from a source is due not so much to 

 the enfeeblement of the individual pulses as to their wider separation 

 from each other, just as on the emission theory the energy of the 

 individual particles does not decrease as the light spreads out; the 

 diminution of the intensity of the light is produced by the spreading 

 out of the particles. 



The idea that bodies are connected by lines of electric force with 

 invisible masses of ether has an important bearing on our views as to 

 the origin of force and the nature of potential energy. In the ordi- 

 nary methods of dynamics a system is regarded as possessing kinetic 

 energy which depends solely upon the velocities of the various parts 

 of which it is composed, and potential energy depending on the rela- 

 tive position of its parts. The potential energy may be of various 

 kinds ; thus we may have potential energy due to gravity and poten- 

 tial energy due to stretched springs, or electrified systems, and we 

 have rules by which we can calculate the value of these potential 

 energies corresponding to any position of the system. When we know 

 the value of the potential energy the method known as that of " La- 

 grange's equations " enables us to determine the behavior of the sys- 

 tem. As a means of calculation and investigation this use of the 



