ek er 
(OE) 
9 
=~ zv Me, 
but 
—4nv?M.,. 
For our purpose this is of no consequence. We shall confine 
ourselves to the case of molecules with a single movable ion or a 
single vector P, and even if we were to consider a more general 
case, our conclusions would not be materially altered. 
S 7. PorNcARÉ investigates the propagation of plane waves in 
the direction of the axis of z. He introduces no resistance, but he 
assumes the existence of several ions in each molecule. 
In his paper (X, Y, Z) denotes the total , dielectric polarization,” 
(Xn, Yn, Zj) one of its parts, (f, g, h) the dielectric displacement. 
His equations, if written partly in the above notations, are 
pee To ae 
POD 1 Yi, Zh 
7 _ (‘ : me B) , ete, (15) 
with the constants A, , Lj and «,, and!) 
af Si er NX 
de v2 ge vw ot ’ 
Ge a ges dn Ero as aoe fo PO 
de ve ot? ve of’ 
ht+Z=0 
Now, if 6 and & do not contain x and y, we shall have by our 
equations (1') and (3') 
0 Dy d Dx 0 Ax d Dy 0D: 
en — <———Agq Ag) 
3 zn dt 82 ee dt 
and 
EC 5 ed 
dz dt dz dt 
1) By a typographical error, the formula of Poincaré which corresponds to the 
first two of the equations (16) has on the left-hand side the sign +, 
