210 Dr. Brewster on the laws of polarisation, &c. 
160 Muriate of lead manganese 
Sulphate of ammonia and — . alumine 
magnesia Specular iron 
red oxide of 165 Pargasite 
Section II. On crystals with one apparent axis of 
polarisation. 
Having thus distinguished the crystals which possess 
double refraction, from those which are destitute of this pro- 
perty, we shall now proceed to determine their optical 
structure, or to ascertain whether the forces which act upon 
the extraordinary ray emanate from one or more axes. 
If we transmit polarised light through any of the parallel 
surfaces of the primitive hexaedral prism of a crystal of 
beryl, and analyse the emergent pencil by a prism of calca- 
reous spar, having its principal section placed either parallel 
or perpendicular to the plane of primitive polarisation, it will 
be found that, when the axes of the prism of beryl is inclined 
45 0 to the plane of primitive polarisation, the vanished image 
is restored. If the thickness of the crystal exceeds 0.035 of 
an inch, the restored light will be nearly white. 
But if the polarised light is transmitted along the axis of 
the prism of beryl, there will be seen a series of beautiful 
circular concentric rings whose centre is the intersection of 
the arms of a dark rectangular cross, and containing all the 
tints in Newton's table of the colours of thin plates. These 
rings, which are shown in Plate xv. fig. 1, increase in diameter 
as the length of the prism is diminished ; but they may be 
distinctly seen, even in the longest prism of beryl. If we 
now examine the double refraction of beryl by transmitting 
