( 54 ) 
vector P;, — which are to be considered as the equations of motion 
or the ponderable matter. They are of the form 
dU, 
dt 
dW ce Vi 
+ dh tnt ise = + gh (B ET alens ==), eta, Ey en 
in which d, f and g are constants. The terms with the first coeffi- 
cient depend on the elastic forces acting in the ponderable particles, 
the terms with f serve to introduce a resistance and consequently 
an absorption of light, while the terms with g are due to the forces 
produced by the magnetic field. 
The field is supposed to be homogeneous; the components of the 
magnetic force in the field are A, B, C. 
In the simplest case there is only one vector P. The signs of sum- 
mation (in (2)) and the indices 4 are then to be omitted, and there 
will be no more than three equations (4). 
§ 3. On the basis of the electromagnetic theory of light I have 
established the equations of motion in the following way *). 
Let there be N equal molecules per unit of volume, each of them 
containing a movable ion of charge e and effective mass z. Let x, y, z 
be the displacements, in the directions of the axes, of one of the 
ions; then ex, ey, €2 wiJl be the components of the electric moment 
of a molecule, and, if a horizontal bar over the letters is employed 
to indicate mean values taken for a large number of particles, the 
components of the electric moment per unit volume will be 
Meier, M, == Ney, MANE, 
If the ions are in a state of vibration, they will excite in the 
aether a certain periodic dielectric displacement and a similar mag- 
netic force; besides these, there may exist, independently of the tons, 
a disturbance of the equilibrium in the aether, in which there is a 
dielectric displacement, say (f), Yo, /o)- 
Now, in order to obtain the equations of motion for one of the 
ions, I conceived a sphere B, whose radius, though very small in 
comparison with the wave-length, is very much larger than the 
1) The sign vete.” will always be used to indicate two equations similar to the 
one that is written down and relating to the axes of y and z. 
2) Lorentz, La théorie Clectromagnétique de Maxwiu1 et son application aux corps 
mouvants, Leiden, Britt 1892. Also in Arch, néerl. T. 25. 
