transmitted through cfystallized Bodies. 203 
about of an inch thick, and has two natural faces which 
are parallel and highly polished. Its refractive power is 1.636 ; 
its dispersive power 0.024, angle at which it polarises 
light by reflection 58° 8'. It is represented in section by ABaby 
in Plate V., fig. 8, DE being one of its depolarising axes. If 
a beam of common light RR' is now incident on the anterior 
surface AB at an angle of about 60° 38', a part of the beam 
will penetrate the topaz at R, and after reaching the posterior 
surface aby it will be partly transmitted at C in the direction 
CF, and partly reflected in the direction Cr, so as to depart 
from the point C almost wholly polarised by reflection; but 
in its passage from C to r along the oblique depolarising axis 
of the crystal, it is depolarised and emerges at r, in the direc- 
tion rr' deprived of the polarity which it had acquired by re- 
flection at C. If the observer now looks into the topaz in the 
direction r'r, through a plate of agate having its laminae 
perpendicular to the plane of the section ABbUy he will per- 
ceive about ten brilliantly coloured elliptical rings, four of 
which, with the two central spots, are shewn in Plate VI., 
fig. 1.* 
The following measures will convey a correct idea of their 
form and magnitude. 
Breadth of the central spots including half the black 
space between them - - - “ 5 ^' 
Distance of the outsides of the central spots - 3 42 
Transverse length of each central spot - “57 
Extreme conjugate diameter first red ring - 7 24 
* I have counted fourteen of these rings when the light was polarised by oblique 
transmission through a plate of mica 0.127th of an inch thick. The colours are iu 
this case much more distinct. 
D d 2 
