oothed, if one rubs it over
with a little oil or white of egg, it becomes quite transparent, so
that the refraction is discerned in it quite distinctly. And this aid
is specially necessary when it is wished to polish the natural
surfaces to remove the inequalities; because one cannot render them
lucent equally with the surfaces of other sections, which take a
polish so much the better the less nearly they approximate to these
natural planes.

Before finishing the treatise on this Crystal, I will add one more
marvellous phenomenon which I discovered after having written all the
foregoing. For though I have not been able till now to find its cause,
I do not for that reason wish to desist from describing it, in order
to give opportunity to others to investigate it. It seems that it will
be necessary to make still further suppositions besides those which I
have made; but these will not for all that cease to keep their
probability after having been confirmed by so many tests.

[Illustration]

The phenomenon is, that by taking two pieces of this crystal and
applying them one over the other, or rather holding them with a space
between the two, if all the sides of one are parallel to those of the
other, then a ray of light, such as AB, is divided into two in the
first piece, namely into BD and BC, following the two refractions,
regular and irregular. On penetrating thence into the other piece
each ray will pass there without further dividing itself in two; but
that one which underwent the regular refraction, as here DG, will
undergo again only a regular refraction at GH; and the other, CE, an
irregular refraction at EF. And the same thing occurs not only in this
disposition, but also in all those cases in which the principal
section of each of the pieces is situated in one and the same plane,
without it being needful for the two neighbouring surfaces to be
parallel. Now it is marvellous why the rays CE and DG, incident from
the air on the lower crystal, do not divide themselves the same as the
first ray AB. One would say that it must be that the ray DG in passing
through the upper piece has lost something which is necessary to move
the matter which serves for the irregular refraction; and that
likewise CE has lost that which was necessary to move the matter
which serves for regular refraction: but there is yet another thing
which upsets this reasoning. It is that when one disposes the two
crystals in such a way that the pla