lready reported.

It was after having explained the refraction of ordinary transparent
bodies by means of the spherical emanations of light, as above, that I
resumed my examination of the nature of this Crystal, wherein I had
previously been unable to discover anything.

18. As there were two different refractions, I conceived that there
were also two different emanations of waves of light, and that one
could occur in the ethereal matter extending through the body of the
Crystal. Which matter, being present in much larger quantity than is
that of the particles which compose it, was alone capable of causing
transparency, according to what has been explained heretofore. I
attributed to this emanation of waves the regular refraction which is
observed in this stone, by supposing these waves to be ordinarily of
spherical form, and having a slower progression within the Crystal
than they have outside it; whence proceeds refraction as I have
demonstrated.

19. As to the other emanation which should produce the irregular
refraction, I wished to try what Elliptical waves, or rather
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