with respect to rays of light. I have even been under some necessity
to make this research, because the refractions of this Crystal seemed
to overturn our preceding explanation of regular refraction; which
explanation, on the contrary, they strongly confirm, as will be seen
after they have been brought under the same principle. In Iceland are
found great lumps of this Crystal, some of which I have seen of 4 or 5
pounds. But it occurs also in other countries, for I have had some of
the same sort which had been found in France near the town of Troyes
in Champagne, and some others which came from the Island of Corsica,
though both were less clear and only in little bits, scarcely capable
of letting any effect of refraction be observed.
2. The first knowledge which the public has had about it is due to Mr.
Erasmus Bartholinus, who has given a description of Iceland Crystal
and of its chief phenomena. But here I shall not desist from giving my
own, both for the instruction of those who may not have seen his book,
and because as respects some of these phenomena there is a slight
difference between his observations and those which I have made: for I
have applied myself with great exactitude to examine these properties
of refraction, in order to be quite sure before undertaking to explain
the causes of them.
3. As regards the hardness of this stone, and the property which it
has of being easily split, it must be considered rather as a species
of Talc than of Crystal. For an iron spike effects an entrance into it
as easily as into any other Talc or Alabaster, to which it is equal in
gravity.
[Illustration]
4. The pieces of it which are found have the figure of an oblique
parallelepiped; each of the six faces being a parallelogram; and it
admits of being split in three directions parallel to two of these
opposed faces. Even in such wise, if you will, that all the six faces
are equal and similar rhombuses. The figure here added represents a
piece of this Crystal. The obtuse angles of all the parallelograms, as
C, D, here, are angles of 101 degrees 52 minutes, and consequently
the acute angles, such as A and B, are of 78 degrees 8 minutes.
5. Of the solid angles there are two opposite to one another, such as
45