C and E, which are each composed of three equal obtuse plane angles.
The other six are composed of two acute angles and one obtuse. All
that I have just said has been likewise remarked by Mr. Bartholinus in
the aforesaid treatise; if we differ it is only slightly about the
values of the angles. He recounts moreover some other properties of
this Crystal; to wit, that when rubbed against cloth it attracts
straws and other light things as do amber, diamond, glass, and Spanish
wax. Let a piece be covered with water for a day or more, the surface
loses its natural polish. When aquafortis is poured on it it produces
ebullition, especially, as I have found, if the Crystal has been
pulverized. I have also found by experiment that it may be heated to
redness in the fire without being in anywise altered or rendered less
transparent; but a very violent fire calcines it nevertheless. Its
transparency is scarcely less than that of water or of Rock Crystal,
and devoid of colour. But rays of light pass through it in another
fashion and produce those marvellous refractions the causes of which I
am now going to try to explain; reserving for the end of this Treatise
the statement of my conjectures touching the formation and
extraordinary configuration of this Crystal.
6. In all other transparent bodies that we know there is but one sole
and simple refraction; but in this substance there are two different
ones. The effect is that objects seen through it, especially such as
are placed right against it, appear double; and that a ray of
sunlight, falling on one of its surfaces, parts itself into two rays
and traverses the Crystal thus.
7. It is again a general law in all other transparent bodies that the
ray which falls perpendicularly on their surface passes straight on
without suffering refraction, and that an oblique ray is always
refracted. But in this Crystal the perpendicular ray suffers
refraction, and there are oblique rays which pass through it quite
straight.
[Illustration]
8. But in order to explain these phenomena more particularly, let
there be, in the first place, a piece ABFE of the same Crystal, and
let the obtuse angle ACB, one of the three which constitute the
equilateral solid angle C, be divided into two equal parts by the
46