transmitted through crystallised Bodies, 207 
If the emergent rays rr', instead of being transmitted through 
agate or Iceland spar, are reflected at the polarising angle 
from any transparent body having its reflecting surface pa- 
rallel to the plane of the section ABa6, they will exhibit the 
first set of rings ; but if the reflecting surface is perpendicular 
to the plane of the section, the second set of rings will be visible. 
When the first set, thus seen by reflection, is examined through 
a prism of Iceland spar, it suffers no change either in the first 
or second image. 
In these experiments X\\e first set of rings is extremely dis- 
tinct, as the polarising crystal extinguishes the light RS re- 
flected from the first surface of the topaz ; but the second set 
of rings is very faint, as the light RS is not extinguished by 
the polarising body. 
When we examine the transmitted light CF, either with 
the naked eye or with polarising crystals, no coloured fringes 
are visible. 
Such are the modifications which common light undergoes 
in its passage through topaz. The affections' of polarised light, 
which now come under consideration, are still more varied and 
interesting. In my first experiments on this subject, I pola- 
rised the light by transmitting it through the agate; but I 
afterwards found it most convenient to, communicate this pro- 
perty by reflection from the surface of a transparent body. 
Let RR', Plate V., fig. 9, be a beam of polarised light ob- 
tained by reflection from any transparent body GH, the plane 
of which it is polarised by oblique transmission through transparent bodies. Hence, 
in all my experiments on the coloured rings, I find it of incalculable advantage to 
polarise the light by bundles of glass plates, and to use them in every case where 1 
formerly employed agate or calcareous spar. 
