37^ Mr Herschel’s Experiments on Circular Polarisation. 
The faces in question,” says Mr Herschel, originate in 
those laws of decrement which Haiiy has called intermediate. 
The primitive form of quartz is a rhomboid, slightly obtuse, 
whose axis is parallel to that of the hexagonal prism. The 
subtractive molecule by which the decrement on the angles E 
(Fig. 8.) takes place, to produce the faces a?, is composed of 
eight of these rhomboids, its edges consisting respectively of one, 
two and four edges of the primitive rhomboid ; and the decre- 
ment resulting is represented in Haiiy’s notation by (E^D^D^). 
The alternate faces of arise from a dilferent law, (as they obvi- 
ously must, the angles upon which they are produced being dif- 
ferently related to the superior vertex). Their law of decre- 
ment cannot be reduced to an integer expression, but is repre- 
sented by (I E^D^B^), in the same notation 
Upon cutting plates perpendicular to the axis of this interest- 
ing variety of rock-crystal, Mr Herschel found that the direc- 
tion of the polarisation was constantly the same as the direction 
of the plagiedral planes, the polarisation being direct or retro- 
grade^ according as these faces leaned forward or backward 
round the summit. The number of crystals which he exa- 
mined amounted to no fewer than twenty-three^ and in all these, 
without a single exception^ the direction of the polarisation was 
the same as the direction of the faces, although M. Biot,” as 
Mr Herschel remarks, has assured us that no peculiarity in 
the crystalline form can lead us to conjecture what may prove 
the direction of rotation in a given specimen of rock-crystal pre- 
vious to trial.” 
Notwithstanding the generality of the fact discovered by Mr 
Herschel, he observed in the possession of Mr Brooke a crystal 
of quartz, which exhibited on one and the same angle of the 
prism plagiedral faces perfectly distinct and in contact, but 
leaning opposite ways round the summit. We trust that Mr 
Brooke will sacrifice this specimen, or rather a part of it, to 
the good of science, and enable Mr Herschel either to esta- 
blish or overturn the important relation which he has discover- 
ed. We venture to say, .with confidence, that this crystal is a 
See Haliy’s Traite de Mineralogies 4;to, Plate 45. and tom. ii. p. 297, 
