REPORT ON MINERALOGY. 
339’ 
to polarize light by transmission. But it appears that these 
are merely instances of a more general fact: many doubly re¬ 
fracting crystals, perhaps all coloured ones, affect the ordinary 
and extraordinary pencils with different colours. Thus beryl, 
by Sir David Brewster’s experiments, when exposed to polarized 
light, transmitted different colours (blue and greenish white), 
as the axis of the crystal was perpendicular or parallel to the 
plane of polarization. 
Other species of mineral crystals were found to possess 
similar properties, and biaxal crystals exhibit it also with certain 
modifications. Sir David Brewster’s list of cases is, as usual, 
considerable. He found also that many minerals absorb certain 
portions of common light, the transmitted portion being more 
or less polarized; so augite, epidote, produce upon light an 
effect partly of the same kind as tourmaline. Smoky quartz 
produces the effect strongly; but it is to be observed that a 
prism of quartz and one of tourmaline polarize in planes, the 
one at right angles to the axis, and the other parallel to it. 
Babinet has recently (1832,) enunciated as the general rule of 
such cases, that one or the other occurs as the crystal is of the 
attractive or repulsive class: but as it appears in fact by Sir 
David Brewster’s previous researches, that the results in the two 
positions do not differ as dark and bright merely, but occur by a 
selection of colours, the general rule thus asserted must require, 
if true, to be differently expressed. 
After these discoveries concerning the optical structure of 
crystalline substances, we might have here supposed that we 
could form some conception of the extent of the variety of na¬ 
ture in this class of phasnomena. In such cases, however, na¬ 
ture is more fertile than our conjectures. It was soon found 
that many crystals possessed a structure far more complex than 
the mere number of axes of a single crystal could give them. 
This discovery also, and the accumulation of cases in which it 
is exemplified, are due to Sir David Brewster. It appears from 
his researches that many kinds of crystals must be considered 
as composed of a most curious mosaic work of crystals, in va¬ 
rious positions, arranged in an order highly complex yet per¬ 
fectly symmetrical. Thus he found in 1817, and announced in 
1819, that amethyst consisted of different portions, which act 
differently on light in an alternate and complementary man¬ 
ner; these portions being generally wedges, with their vertices 
towards the axis of the crystal, or a series of V’s one within an¬ 
other, exhibiting the outlines of such w 7 edges. 
Again, it appeared that apophyllites from Iceland and from 
Fer roe were composed of a most curiously tessellated structure, 
v 2 
1 /V 
