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.