Dr. Brewster on the laws of polarisation , &c. 221 
position, one of the arms, MN, being always in the plane of 
primitive polarisation, and the other, PR, in a plane perpen- 
dicular to it. Hence it follows, that whenever a plane pas- 
sing through the axis of the crystal, is either in the plane of 
primitive polarisation, or perpendicular to it, the extraordi- 
nary pencil or tint disappears. This result will be found 
correct, whatever be the direction in which the polarised ray 
traverses the crystal, and is therefore the general law of the 
disappearance of the extraordinary pencil for crystals with 
one axis. 
Section 111. On crystals with two or more axes of polarisation. 
The complicated structure of mica was first discovered in 
1812 by M. Biot and myself, without any knowledge of each 
others observations.. M. Biot considered one kind of mica as 
the only mineral known to possess this compound structure, 
indicating the existence of two axes ; whereas, so early as the 
beginning of 1813, I had found the same structure in topaz * 
nitre, sulphate of potash, tartrate of potash and soda, acetate of 
lead, and mother of pearl ; and I have since discovered that 
this is the general structure of doubly refracting crystals. 
The following is a list of crystals, which I have found to 
possess more than one axis of polarisation and double re- 
fraction. 
Arragonite 
Mica 
Topaz 
Feldspar 
5 Stilbite 
Cymophane 
Axinite 
Olivine 
* The drawings of the rings in topaz, which I published in the Phil. Trans, for 
1814., are an accurate representation of the phenomena,' very near the resultant axes 
of crystals with more than one axis. „ 
