transmitted through crystallized Bodies. 195 
when the ray RS is parallel to nin, and a ray PQ will, within 
certain limits, suffer less diminution of lustre than a ray TV 
falling with the same angle of incidence on the other side 
of RS. 
When RS and mn are perpendicular to the surfaces of AB, 
and when PQ and TV form equal angles with the perpendi- 
cular RS, their intensity should be equal ; but this is by no 
means the case, for the transmitted light which is incident on 
the side T of RS appears to have a different character from 
that which is incident on the other side. We must therefore 
suppose that there is some other peculiarity of structure in the 
agate, connected probably with that particular kind of crys- 
tallisation which polarises light, to which this curious fact 
must be ascribed.^ 
The intensity of the light transmitted by the agate is like- 
wise affected by its polarising property. If a ray Rr, Plate V., 
fig. 4, is incident upon a piece of agate AB, so as to be polarised 
by reflection from the second surface, then, since it is polarised 
during its passage from r to E, the bright image will suffer 
total reflection at E, while the nebulous image will be trans- 
mitted like common light in the direction EG. If the agate is 
now turned round 90" the nebulous image will suffer total 
reflection at E, while the bright image will penetrate the se- 
cond surface at E like common light. When the incident ray 
Rr has different obliquities and the agate intermediate posi- 
tions, the intensity of the transmitted light will be more or 
less affected by its polarising power. 
The preceding observations on the laminated structure of 
♦ See Edinburgh Transactions, Vol. VII. Part II. 
C C 2 
