in the Optical Phenomena of Mother of PeaH. 417 
but ill a manner opposite to rS like the other pencil in calca- 
reous spar : or if the ray Rr is transmitted through a bundle 
of glass plates, the whole of the pencil rT will be polarised 
in that manner. 
If we now suppose AB a single plate of mother of pearl 
about one-fortieth of an inch thick, and the ancrle of incidence 
R''C about 60°, the reflected ray rS will be polarised as in 
every other transparent body ; but the transmitted ray rT will 
he wholly polarised, and in the same manner as the refected ray 
rS, while in every other transparent body that has been exa- 
mined the ray rT possesses an opposite kind of polarisation. 
If we turn the plate AB round its centre r, so as to preserve 
its inclination to the incident ray Rr, no change whatever takes 
place, the transmitted ray still retaining its former polarity. 
The angle of incidence RrC, at which the transmitted light 
rT is wholly polarised, varies in the inverse ratio of the thick- 
ness of the plate AB, and the whole pencil is polarised at any 
angle greater than that angle. The relation between the 
angle of polarisation and the thickness of the plate remains to 
be determined; though I suspect it will be found that the tan- 
gents of the angles of incidence at which the whole of the 
pencil is polarised, are inversely as the thickness of the plates. 
The phenomena which I have now described, I have ob- 
served in every piece of mother of pearl that I have tried ; 
and as they are not affected when the incident pencil is 
refracted from balsam of Tolu, or any other cement, into the 
mother of pearl, they are obviously unconnected with its su- 
perficial configuration. Ivory does not produce the same effect 
upon light. 
From these results the following conclusions are clearly 
deducible. 
