4 Dr. Brewster on the Optical Properties 
“angles of 20°+. In these the central portion is often of a 
“high yellow colour, and manifests the phenomenon of absorp- 
“tion of polarized pink and yellow rays alternately, while the 
“‘ external border is nearly colourless in all positions, and pre- 
* sents no such phenomenon.” 
The tesselated structure is so common in the Brazilian 
Topaz, that I am more disposed to regard it as an _ essential 
character of that mineral, than as an accidental formation. Out 
of the great numbers which I have examined, there is not one 
in an hundred which is free from this hemitropism; and in some 
very fine crystals, in the examination of which this structure 
had escaped my notice, I have since detected it by more careful 
methods of observation. In these cases, however, the crystal is 
rather to be considered as a compound than as a _ hemitrope 
crystal, for the separated portions have their principal sections 
much nearer to coincidence, than in less perfect specimens. 
The hemitropism of the Brazilian Topaz is of a very sin- 
gular kind. The tesselae are not turned round one half or any 
determinate portion of a circle, as this term implies, but the 
principal sections of different laminz form different angles with 
one another, and hence we may distinguish these specimens by 
the more appropriate name of polytrope crystals. 
This curious formation, which has not hitherto been observed 
in doubly refracting structures, will be understood from Fig. 14, 
where ABED, CBEF are the two external tessela at one of 
the obtuse angles of the rhomboid. If we suppose that these 
tessela are divided into four lamine, 1, 2, 3, 4, and that MN 
is the principal section, or one of the neutral axes of the central 
portion of the crystal contiguous to DEF, then the laminz 1, 1, 
have their principal section in the direction aa’ forming a very 
small angle with MMW; the lamine 2,2 have their principal 
section in the line 64’, and so on to the superficial lamine 4, 4, 
