338 Mr Herschel on the Deviations from Newtons Scale 
But it is evident from a moment’s consideration of the forms 
of the three curves representing the polarizing powers of three 
varieties of apophyllite, that no one of them can be produced by 
any combination of the other two according to this law, and we 
are therefore necessitated to admit each as a distinct variety, or 
at least composed of laminae of not fewer than three kinds. This 
alternation or superposition of laminae of different polarizing 
powers is no hypothetical case. I have observed its occurrence, 
not only in the instance before us, but in other crystals of per- 
fect regularity in their external forms. Dr Brewster has also 
observed phenomena referable to this principle in his paper on 
the apophyllite. 
TXyposulphate of lime (formed by the union of that base with 
the hyposulphuric acid lately discovered by Welter and Gay 
Lussac, (See Ann. de Chimie y X. March 1819,) affords another 
instance of deviation from Newton’s Scale in crystals of double 
refraction. This salt crystallizes in bevelled hexagonal tables, 
which have no distinct cleavage, the axis being perpendicular to 
their broad surfaces. The following is the scale of tints de- 
veloped by a plate of this salt on exposure to polarized light. 
I Table VI .—Hyposulphate of Lime. Thickness = 35701 . 
The axis was inclined 5° 12' to the surface in the plane of 
incidence. 
Incidence. 
Ordinary Pencil. 
Extraordinary Pencil. 
O’ O' 
White. 
Black. 
White. 
Very faint sky-blue. 
10° 32' 
Very pale yellow. 
Pretty strong sky-blue. 
Sombre yellow. 
Very light bluish-white. 
Sombre pink 3 r ellow. 
White. 
Sombre purple crimson. 
White. 
15° V 
Beautiful rich dark purple. 
White, a little yellowish. 
Beautiful deep blue. 
Bright straw colour. 
Bright blue. 
. Yellow. 
Fine light blue. 
Yellow verging strongly to orange- 
pink. 
Light greenish blue. 
Fine pink. 
Light yellow green. 
Sombre pink. 
2 V 2 V 
Light greenish-yellow. 
Purple. 
