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.