spheroidal waves, would do; and these I supposed would spread
indifferently both in the ethereal matter diffused throughout the
crystal and in the particles of which it is composed, according to the
last mode in which I have explained transparency. It seemed to me that
the disposition or regular arrangement of these particles could
contribute to form spheroidal waves (nothing more being required for
this than that the successive movement of light should spread a little
more quickly in one direction than in the other) and I scarcely
doubted that there were in this crystal such an arrangement of equal
and similar particles, because of its figure and of its angles with
their determinate and invariable measure. Touching which particles,
and their form and disposition, I shall, at the end of this Treatise,
propound my conjectures and some experiments which confirm them.

20. The double emission of waves of light, which I had imagined,
became more probable to me after I had observed a certain phenomenon
in the ordinary [Rock] Crystal, which occurs in hexagonal form, and
which, because of this regularity, seems also to be composed of
particles, of definite figure, and ranged in order. This was, that
this crystal, as well as that from Iceland, has a double refraction,
though less evident. For having had cut from it some well polished
Prisms of different sections, I remarked in all, in viewing through
them the flame of a candle or the lead of window panes, that
everything appeared double, though with images not very distant from
one another. Whence I understood the reason why this substance, though
so transparent, is useless for Telescopes, when they have ever so
little length.

21. Now this double refraction, according to my Theory hereinbefore
established, seemed to demand a double emission of waves of light,