Mr Herschel on the Double Kef r action of JpophylUte. 137 
the extraordinary ray may be regarded as arising from a repul- 
sive force, emanating from one or more axes, as in carbonate of 
lime ; and the other, from an attractive, as in zircon. Interme- 
diate between these, and forming, as it were, the limit between 
both, are bodies devoid of the property of double refraction, as 
fluor, glassj See* All these substances, however, act with diffe- 
rent degrees of energy on the differently-coloured rays, accord- 
ing to a law which appears subject to great variations in diffe- 
rent bodies, and which is directly deducible in any given body, 
from the series of tints developed by exposure to polarized light. 
In a former paper I have instanced some remarkable devia- 
tions from the ordinary law of tints exhibited by certain varie- 
ties of the apophyllite ; but from the mode of experimenting 
there followed, the most remarkable of the peculiarities present- 
ed by the specimens employed escaped my notice. It is this, 
that out of the three varieties examined, two appear to belong 
at once to all the three classes of Media above enumerated ; po&^ 
sessing the property of attractive crystals when exposed to the 
rays forming one extreme of the spectrum, and of repulsive in 
their action on the other extreme ; while, for certain intermediate 
rays, they are altogether void of the property of double refrac- 
tion, and allow such rays to pass freely through them in all di- 
rections^ without dividing them into two pencils. 
I was led to a knowledge of this remarkable singularity, by 
the consideration of the forms of the curves traced in Figs. 2. 
and 4. *1-, whose ordinates represent the polarizing energies of 
the two varieties in question, on the rays whose place in the 
spectrum is denoted by their abscissae. It is assumed^ without 
any particular grounds, in my former paper, that these curves 
lie, throughout their whole extent, on one side of their abscissae. 
This was natural enough, being accustomed to regard crystals 
as necessarily included in one or the other of the great divisions 
above referred to, and in the mode of experimenting there re- 
sorted to, the contrary could not be discerned, the change from 
attractive to repulsive being marked by no phenomenon. In 
• Transactions of the Philosophical Society of Cambridge, vol. i. part i. 1 820. 
See Plate Vll. of Vol. IV. of this Journal, Fig, 2, and 4. 
