polish it. Cutting is easy by the slicing wheels of lapidaries, or in
the way in which marble is sawn: but polishing is very difficult, and
by employing the ordinary means one more often depolishes the surfaces
than makes them lucent.
After many trials, I have at last found that for this service no plate
of metal must be used, but a piece of mirror glass made matt and
depolished. Upon this, with fine sand and water, one smoothes the
crystal little by little, in the same way as spectacle glasses, and
polishes it simply by continuing the work, but ever reducing the
material. I have not, however, been able to give it perfect clarity
and transparency; but the evenness which the surfaces acquire enables
one to observe in them the effects of refraction better than in those
made by cleaving the stone, which always have some inequality.
Even when the surface is only moderately smoothed, 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,
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