( 01 ) 



the elementary theory by a more complete one. Some time ago, I 

 examined 1) therefore what phenomena are to be expected, if a mole- 

 cule, having an arbritary number of degrees of freedom and arbri- 

 tarilv distributed electric charges, is oscillating about a position of 

 equilibrium. 



Before returning to this subject, I will consider the conclusions 

 which may be obtained by arguments from symmetry, without ente- 

 ring into the details of the mechanism of radiation. 



There can be no doubt, that we may consider the source of light 

 as a system of extremely small particles, oscillating partly with the 

 frequency of the light vibrations; in virtue of their electric charges, 

 these particles must excite in the surrounding aether periodically 

 oscillating dielectric displacements. These constitute the luminous 

 motion radiated by the source. 



For briefness' sake this entire system will be indicated by 8. 



We may now conceive a second system S', which is the image of 

 8, relatively to a fixed plane P. The meaning of this is as follows. 



If A is a particle in S, there is in 8' a particle A', which is 

 the image of A and of the same physical nature as this particle. 

 Especially, the mass and the electric charge are the same ; or, to speak 

 more accurately, in corresponding points of A and A' the same 

 material density and the same density of electric charge will be 

 found. Moreover, the particles A' will be at every moment the image 

 of the particles ^1, or, as we may say, the motion of the ions in 

 8' will be the image of the motion in .S'. If this is the case, the 

 luminous motion in the aether in 8' will likewise be the image of 

 the motion in ,S', in this sense that the vector representing the die- 

 lectric displacement in (S" is always the image of a corresponding 

 vector in 8. 



Of course ail this will only be possible, if the forces operating 

 on the particles A' are the images of those to which the particles 

 A are subjected. So far as the mutual action of the particles is 

 concerned, we may regard this as a consequence of the supposed 

 equality in physical nature. In order that the forces, originated by 

 the external magnetic field, may satisfy the same condition, we will 

 suppose that the vectors, representing the magnetic force in »S", may be 

 derived from the corresponding ones in *S' by first taking their ima- 

 ges, and then reversing the directions of these images*}. 



ij Wied. Ann. Bd. 63, p. 278, 1807. 



2) If the magnetic field is generated by electric currents, we may imagine the 

 required field in S' to be produced by currents, which are the images of the currents in ,S', 



