rule. Let the parallelogram GCFH, made by the principal section of the
Crystal, as previously determined, be traced separately. I found then
that always, when the inclinations of two rays which come from
opposite sides, as VK, SK here, are equal, their refractions KX and KT
meet the bottom line HF in such wise that points X and T are equally
distant from the point M, where the refraction of the perpendicular
ray IK falls; and this occurs also for refractions in other sections
of this Crystal. But before speaking of those, which have also other
particular properties, we will investigate the causes of the phenomena
which I have already 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