Dr. Brewster on the laws of polarisation, &c. 229 
system shown in PL XV. fig. 1, arises from the great attenua- 
tion of the crystallized plate, which is no longer capable of 
developing the tints between the poles of no polarisation ; and 
it is only from the elliptical form of the curves that we can 
in this state recognise the existence of two axes. Even this 
ellipticity will disappear by a farther attenuation of the plate; 
and the crystal deprived as it were of one of its axes, appears 
to act upon light, exactly like beryl or calcareous spar. 
The phenomena which have now been explained, I have 
found, under various modifications which will afterwards be 
described, in all the crystals that have more than one axis. 
The irregularities in the tints are, in every case, developed by 
increasing the thickness of the plate, and affect the rings 
which are formed at a considerable distance from the poles 
of no polarisation. They are analogous to the tints seen 
along the axis of rock-crystal ; and as the secondary forces 
by which they are produced, conceal the regular action of 
the principal forces, we must abstract these secondary effects 
in determining the law according to which the legitimate 
tints are developed. 
2d. On the character, the number, and the position of the axes, 
by which the tints are produced. 
The most important physical circumstance which distin- 
guishes the system of rings formed by different crystals, is 
the magnitude of the arch PP', or the inclination of the resul- 
tant axes, or diameters of no polarisation ; for it is from this 
angle alone that we can deduce the relative intensity of the 
real axes, if we suppose them rectangular ; or their mutual 
inclination, if we suppose them equal and inclined. 
