142 Dr Brewster 07i the Phenome^ia of ApophylUte. 
Observations on the preceding Paper. By Dr Brewster. 
The very important fact established with so much ingenuity 
and address in the preceding paper appeared to me so highly 
interesting, that, in a paper read before the Boyal Society of 
Edinburgh on the 17th December 1821, I have endeavoured to 
refer it to the ordinary influence of three positive rectangular 
axes, which are in equilibrium only for the yellow rays of the 
spectrum. 
In the Biaxal Apophyllite,” as I have remarked in that pa- 
per, one of the polarising axes must he in the plane of the la- 
minae, and in both the Biaxal and the Uniaxal Apophyllite, 
there appears to be an axis perpendicular to the laminae. As 
the form of the prism of Apophyllite is perfectly symmetrical in 
relation to the axis, it is probable that there are two equal and 
rectangular axes, of a Positive or Attractive character, in the 
plane of the laminae, each axis being perpendicular to the paral- 
lel faces of the crystal, and we know that there is a Positive or 
Attractive axis at right angles to the laminae, and coincident 
with the axis of the prism ■[-. The two equal Positive axes, 
which we shall call the Horizontal axes, on the supposition that 
the prism is placed upon its base, will obviously produce a single 
Negative axis, coincident with the other Positive axis, or the 
Vertical one perpendicular to the laminae, and the system of 
rings round the resultant of these two axes, will deviate more 
or less from Newton’s scale, according to the nature of the dis- 
persive forces of the elementary axes j. Let us suppose that 
the resultant negative vertical axis has the same action upon the 
Yellow rays of the spectrum as the real positive vertical axis ; 
• A slight notice of this discovery of Mr Herschel’s was given in this Journal 
Vol. V. p. 213. See also Vol. I. p. 5. 
•j" I must refer the reader to my paper “ On the Laws of Polarisation and 
Double Refraction,” in the Phil. Trans. Lond. for 1818, p. 231, and p. 245,-254. 
for the grounds upon which this resolution of axes is made. In the case of Apophyl- 
lite, there are reasons of a peculiar kind for supposing the existence of three axes. 
For an illustration of these views, the reader is referred to my letter to Mr 
Herschel, published at the end of his paper in the Phil. Trans. 1820, p. 94. ; and 
to Mr Herschel’s paper in the Transactions of the Cambridge Philosophical Society^ 
vol. i. p, 21. ; or in this Journal^ Vol, iv. p. 335. ; and Vol. v. p. 340. 
