4 Dr Brewster on the Structure of Apophyllite. 
tions of one crystal, whereas the combined crystals which form 
a made, preserve each their separate form, and are merely two 
crystals mechanically united. 
These conclusions enable us to explain the uniformity both in 
the colour and character of the tints, over the whole plate MNOP, 
when it is inclined more than 17^ 4' in the plane MN. Be- 
yond this inclination, we observe the tints in RT, SV, without 
the poles of no-polarisation, and therefore they are positive, or have 
the same character as the tints produced by the rectangle abcd^ 
and the rectilineal portions that have only one axis ; — that is, 
these tints will rise into higher orders when crossed with sul- 
phate of lime. In like manner, in the rectangles RV and ST, 
we observe the tints in a plane perpendicular to that of the re- 
sultant axes ; and as these will also rise to higher orders when 
crossed with sulphate of lime, the character of all the tints in 
the plate will be positive, and they will nearly have the same in- 
tensity when the inclination of the plate exceeds half the inclina- 
tion of the resultant axes. 
In examining the action of Apophyllite in planes perpendicu- 
lar to the axis, I had the advantage of very large pyramids 
which Major Petersen, to whom I had exhibited the preceding 
results, brought me from the Faroe Islands in the autumn of 
1817 ; and from the free access which I have at all times had 
to the cabinets of Mr Allan and Sir George Mackenzie, which 
are peculiarly rich in apophyllites, I have been enabled to ob- 
tain results of greater generality and interest. 
Through one of these pyramidal crystals, which is shewn in 
Fig. 7, I observed the two images formed by double refraction, 
and found that the principal axis was positive, and that the 
maximum deviation of the extraordinary ray was much inferior 
to that of Mesotype. Upon transmitting polarised light through 
tv/o of the broadest faces, they appeared covered with fringes, 
as shewn in Fig. 7, those near the summit a b being bluish-violet 
and greenish-yelloK^ and those produced at a greater thickness 
being green and pink. The fringes were bent at the meeting of 
the pyramidal planes, as shewn in the figure ; and, when the 
light was transmitted ihrcmgh the faces VRa c, TSd 6, the same 
fringes appeared j bui the greeii and pink tints commenced at 
