580 Prof. J. Larmor on the Intensity of the 
ee force (0, 0, A cos pt), and therefore magnetic force 
1 (0, A cos pé, 0), the equation of its forced vibration is 
MM 
yp + «?M=eA cos pt, 
so that M= 

BEG COS pt 5 
and, the vibrator constituting a current element dM/dt, the 
magnetic field pushes it along z with a mechanical force 
BdM/dt, which is 
Ome 
P ep cos pt sin pl. 
This electromagnetic force is, however, purely alternating, and 
so adds up in time to nothing : the only way to obtain steady 
mechanical pressure on the vibrator is to put the forced 
vibration out of phase with the exciting field by the intro- 
duction of a frictional term into the equation of vibration, 
which will correspond to opacity. 
In the theory of exchanges of radiation, it is customary to 
represent a perfect reflector as a body of very high electric 
conductivity. .Any body across which the radiation cannot 
penetrate is, as already stated, subject to a pressure from the 
radiation just outside it, determined by Maxwell’s formula. 
It is worth while to verify explicity that the absorbing 
quality which must be associated with this pressure does not 
act so as to vitiate the perfection of the reflexion by degrading 
the energy. This is, of course, readily done. The equations 
of wave-propagation already formulated lead to 
dQ poe na, ay 
Writing Q= uae 
this gives p?=Kyo-?n? + Aarpone. 
Thus, if the conductivity o is largely preponderant we may 
write | 
p=(Qrpno)?(1+2), say =r(1+2). 
Taking the real part * 
Q=Aee-™ cos 705 
* But this is for stationary waves ; it should have been for progressive 
waves Q=Ae-"" cos (nt—7rv), giving 
