REFRACTION AND POLARIZATION. C9 
series of rings are visible only when viewed in directions very 
oblique to one another. 
4, Circular Polarization in Uniaxial Crystals.—It is stated 
on page 53 that quartz crystals have often a left-handed and a 
right-handed arrangement of planes. This is connected with a 
right-handed and left-handed molecular structure in crystals of 
this species. When a plate cut at right angles to the axis is ex- 
amined by the polarized light, instead of presenting a black 
cross, the centre of the rings appears brightly colored, and if the 
polarizer is revolved, this color changes from blue to yellow, 
then red, right-handed crystals requiring revolution to the 
right and left-handed to the left for this succession. This prop- 
erty seems to distinguish the smallest grains of quartz, and may 
be easily observed in a good polariscope. 
5. Anomalies in Polarization.—There are some isometric 
erystals which have the property of polarization. LBoracite is 
one example; and it is explained by the presence of another 
mineral in minute particles, distributed regularly through the 
crystals. Perofskite is another case; and it has suggested a 
doubt as to its being isometric. Octahedrens of alum some- 
times have polarization, and it has 
been shown to be due to the crystals 
being made up of thin plates—light, 
when transmitted through a pile of 
such plates, becoming polarized. Dia- 
monds are sometimes uniaxial. 
Analcite was long since described 
by Sir David Brewster as an example 
of polarization under the isometric 
system. Its trapezohedrons exhibit 
u symmetrical arrangement of lines of 
prismatic colors and alternating dark 
lines with cross-bands, as imperfectly 
shown in the annexed figure. Tvapezohedrons of leucite are 
somewhat similar in their polarizing character. The efiect in 
both species 1s connected with twinning; but, besides, accord- 
ing to recent observers, the crystallization is dimetric. One 
writer makes crystals of analcite to be trimetric twins, analogous 
those of phillipsite. Twinning in crystals is a very common 
pource of irregularities. A regular twinning of lamine of bi- 
axial crystals around a centre may give a uniaxial character to 
the twin. Apophyllite is a dimetrie species, showing peculiari- » 
ties in its colors arising from the different action of the mineral 
in light of different colors. 















