( 324 ) 
' ( n +'^)< n - nW )-^ 
and for Q 2 - )_ and Q 2 ~ we have similar expressions, containing sums 
which relate to the modes of motion je 2 + and x 2 _. The symbols 
2, 2, 2 serve to indicate sums that must be extended either 
*1 *2+ *2- 
over the modes *j, or over the modes «2+ or 9t 2 _ of one of the 
groups a, b ,and all these groups have their share in the sums 
S. The coefficient g, which we shall suppose to have the same value 
ab.. 
for all terms belonging to one and the same group, represents the 
influence of the resistances, and may be regarded as a measure of 
the breadth of the absorption lines. As to the quantities these 
are all real and positive; their value depends on the structure of 
the particles and is the same for two conjugate vibrations. 
Let us denote by £> the dielectric displacement, so that 
P. • ....... (2) 
and let us abbreviate by putting 
* + Qi = ■ 
1 + Q2++Q 2 _ = S 2 , 
Q2+ - Q 2 _ = R. 
Then we have the relations 
4; 
—.(3) 
£>* = Si <£ Zi ] 
which are to be combined with the general equations of the electro¬ 
magnetic field 
(4) 
rot<£ = — ijj>. . ..(5) 
In these latter formulae £ denotes the magnetic force that belongs 
to the beam of light and alternates with its frequency. 
§ 3. We shall examine the propagation of plane waves in a 
direction lying in the plane xz, and making a positive sharp angle 
# with the axis of z. Let the variable quantities which determine 
the state of the system contain the coordinates and the time only 
in the factor 
