( 
) 
the fact that the principal beams are not circularly, but elliptically 
polarized. 
Let one of the beams be represented by 
and tli 
one bea 
other by similar expressions containing instead of 
> instead of + tu. Or, in other terms, Jet us write for 
and for the other 
-fH- 
Then, compounding the two, and putting 
* = 5 to- - m-)» 
we get 
= 2a «-** cob (** 1 + m) cos £ n, t - ^ (n+ + (*_) *' + J>j , 
£),j = 2a r-V ms xpz' ms j^n 0 1 — (fi + + (*-) *'+?"]• 
Hence, at any point z\ the resultant vibration is rectilinear, and 
its amplitude, considered as a vector, may be represented by 
2 a e~ hz ' 31 , 
21 being a vector in the wave front with the components 
'!»' = cos (ip 2 ' 4- u>) , 21 y = cos ip z. 
It this vector is drawn from a fixed point, its extremity describes 
an ellipse when z ' is made to increase continually. The corresponding 
rotation of the plane of polarization is similar^ to the one observed 
in more familiar cases inasmuch as it goes on in a constant 
direction, but when z' is made to increase at a constant rate, the 
velocity of the rotation is variable. Its changes are determined by 
