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 planes which constitute the principal
sections intersect one another at right angles, whether the
neighbouring surfaces are parallel or not, then the ray which has come
by the regular refraction, as DG, undergoes only an irregular
refraction in the lower piece; and on the contrary the ray which has
come by the irregular refraction, as CE, undergoes only a regular
refraction.
But in all the infinite other positions, besides those which I have
just stated, the rays DG, CE, divide themselves anew each one into
two, by refraction in the lower crystal so that from the single ray AB
there are four, sometimes of equal brightness, sometimes some much
less bright than others, according to the varying agreement in the
positions of the crystals: but they do not appear to have all together
more light than the single ray AB.
When one considers here how, while the rays CE, DG, remain the same,
it depends on the position that one gives to the lower piece, whether
it divides them both in two, or whether it does not divide them, and
yet how the ray AB above is always divided, it seems that one is
obliged to conclude that the waves of light, after having passed
through the first crystal, acquire a certain form or disposition in
virtue of which, when meeting the texture of the second crystal, in
certain positions, they can move the two different kinds of matter
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