60 



assuming the vibration!? of a polarized ray to be perpendicular to 

 the plane of polarization. 



The well-known condition that a plane polarized ray may be 

 transmissible without subdivision, and the velocity of propagation 

 may be immediately deduced from this equation. 



if we assume the vibrations of a polarized ray to be in the plane 

 of polarization, the equation becomes 



~ =-D4D.(« l -aAa + ^/3d^ + c-yA 7 )UlD v. 



This includes Professor MacCullagh's three equations. 



