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,
both of them spherical (for both the refractions are regular) and
those of one series a little slower only than the others. For thus the
phenomenon is quite naturally explained, by postulating substances
which serve as vehicle for these waves, as I have done in the case of
Iceland Crystal. I had then less trouble after that in admitting two
emissions of waves in one and the same body. And since it might have
been objected that in composing these two kinds of crystal of equal
particles of a certain figure, regularly piled, the interstices which
these particles leave and which contain the ethereal matter would
scarcely suffice to transmit the waves of light which I have localized
there, I removed this difficulty by regarding these particles as being
of a very rare texture, or rather as composed of other much smaller
particles, between which the ethereal matter passes quite freely.
This, moreover, necessarily follows from that which has been already
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