in Media of every degree of Transparency. 109 
treat as functions of the codrdinates of the center of the sphere 
a 
values of €, 7, €. But however they may be designated, it is 
essential to remember that it is a space-average for a certain 
. Let us suppose that luminous vibrations of any one 
tiod* are somewhere excited, and that the disturbance is 
propagated through the medium. The motions which are 
excited in any part of the medium, and the forces by which 
they are kept up, will be expressed by harmonic functions of 
the time, having the same period,+ as may be proved by the sin- 
gle principle of the superposition of motions, quite independ- 
ently of any theory of the constitution of the medium, or of 
the nature of the motions, as electrical or otherwise. This is 
equally true of the actual motions, and of the averages which 
Wwe are to consider. We may therefore set 
27 ss. 
LSJave = a, cos ry + @, sin a ; (1) 
etc., 
ee denotes the time, p the period, and a,, a,, functions of 
the codrdinates. It follows that 
“i 47? 
[Slave = - [ laves (2) 
ete. 
hy There is no real loss of generality in making the light monochromatic, since 
every case it may be divided into parts, which are separately propagated, and 
ont * 18 of course possible that the expressions for the forces an 
we d have constant terms ut these will disappear, if the displacements are 
from the state of equilibrium about which the system vib 
ave out of account in measuring the forces (and the electrostati 
in the restricted sense of dielectric displacement or polarization : 
i d in this paper, constitutes wha’ 
a 
ac t, as the term is us 
Well calls the total motion of electricity or tru 
i t ur 
we with reference to the total motion of electricity in a mann 
a to that in which the term is ordinarily used in the theory of wave- 
