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
straight line CG, and let it be conceived that the Crystal is
intersected by a plane which passes through this line and through the
side CF, which plane will necessarily be perpendicular to the surface
AB; and its section in the Crystal will form a parallelogram GCFH. We
will call this section the principal section of the Crystal.

9. Now if one covers the surface AB, leaving there only a small
aperture at the point K, situated in the straight line CG, and if one
exposes it to the sun, so that his rays face it perpendicularly above,
then the ray IK will divide itself at the point K into two, one of
which will continue to go on straight by KL, and the other will
separate itself along the straight line KM, whi