Dr. Brewster's experiments on the 
vanished image will begin to appear, and when MO is in the 
plane of reflection RrS, it will have reached its maximum 
brightness. It will again vanish when OP is in the plane of 
reflection, and will again recover its lustre when ON is in 
that plane, having vanished and reappeared four times in 
the course of one revolution of the rhomb. If the rhomb 
MNOP is kept fixed when the vanished image has reap- 
peared ; and if the prism CD is turned round, the two images 
will continue visible during every part of its circular motion, 
and hence the polarised ray rS, seems to have been robbed of 
its polarisation or depolarised. 
In order to explain these appearances, let CD be fixed in 
its former position, and let the rhomb MNOP have its prin- 
cipal section or neutral axis in the plane RrS. This rhomb 
is known to give two images of the candle formed by rays 
ST, SV, nearly coincident, but owing to its present position, 
one of the pencils, that would have moved in the direction 
ST, refuses to penetrate the rhomb, and therefore only one 
pencil SV, polarised in the same manner as rS, falls upon the 
prism CD. Now this prism being obviously placed in the 
position where its power of doubling SV is extinguished, that 
is, where one of the pencils, into which it separates SV, 
has vanished, a single image E of the candle will still only 
be visible, notwithstanding the interposition of the rhomb. 
The very same reasoning is applicable to the case where the 
longer diagonal OP is in the plane of reflection. 
The prism CD continuing fixed, as before, let the side MO, 
or the depolarising axis of the rhomb, be brought into the 
plane of reflection. In this situation of the crystals, both the 
pencils ST, SV, fall upon the prism CD, which has now the 
