JEther "Drift " and Rotary Polarization. 



389 



slight increment, say ; then, if it be reflected back, there would 

 be a corresponding decrement over what would occur if either 

 there were no motion or the direction of propagation were 

 across the drift. Thus in fig. 2, if AB is the initial direction 

 of vibration, and if the active substance rotate it through a 

 whole number of circumferences without motion, it would be 



Fig. 2. 



slightly increased, if propagated along the drift say, and the 

 emergent vibration be in the direction of A' B'. If now this 

 ray is reflected back by a single reflector against the drift, it 

 would be rotated in the opposite direction (to a fixed observer) 

 by the same number of circumferences less the angle 

 between AB and A'B', vibrating on emerging in the direction 

 of A^B". Hence wo should have a recom position of all the 

 colours to this extent. If now the optical system be turned 



