llucid when view'd singly,
tho' the Reflexions in their cylindrical Surfaces may make the whole
Nerve (composed of many Capillamenta) appear opake and white. For
opacity arises from reflecting Surfaces, such as may disturb and
interrupt the Motions of this Medium.

[Sidenote: _See the following Scheme, p. 356._]

_Qu._ 25. Are there not other original Properties of the Rays of Light,
besides those already described? An instance of another original
Property we have in the Refraction of Island Crystal, described first by
_Erasmus Bartholine_, and afterwards more exactly by _Hugenius_, in his
Book _De la Lumiere_. This Crystal is a pellucid fissile Stone, clear as
Water or Crystal of the Rock, and without Colour; enduring a red Heat
without losing its transparency, and in a very strong Heat calcining
without Fusion. Steep'd a Day or two in Water, it loses its natural
Polish. Being rubb'd on Cloth, it attracts pieces of Straws and other
light things, like Ambar or Glass; and with _Aqua fortis_ it makes an
Ebullition. It seems to be a sort of Talk, and is found in form of an
oblique Parallelopiped, with six parallelogram Sides and eight solid
Angles. The obtuse Angles of the Parallelograms are each of them 101
Degrees and 52 Minutes; the acute ones 78 Degrees and 8 Minutes. Two of
the solid Angles opposite to one another, as C and E, are compassed each
of them with three of these obtuse Angles, and each of the other six
with one obtuse and two acute ones. It cleaves easily in planes parallel
to any of its Sides, and not in any other Planes. It cleaves with a
glossy polite Surface not perfectly plane, but with some little
unevenness. It is easily scratch'd, and by reason of its softness it
takes a Polish very difficultly. It polishes better upon polish'd
Looking-glass than upon Metal, and perhaps better upon Pitch, Leather or
Parchment. Afterwards it must be rubb'd with a little Oil or white of an
Egg, to fill up its Scratches; whereby it will become very transparent
and polite. But for several Experiments, it is not necessary to polish
it. If a piece of this crystalline Stone be laid upon a Book, every
Letter of the Book seen through it will appear double, by means of a
double Refraction. And if any beam of Light falls either
perpendicularly, or in any oblique Angle upon any Surface of this
Crystal, it becomes divided into two beams by means of the same double
Refraction. Which beams are of the same Colour with the incident