ishing the surface
which has been flattened in this manner.

As for the other method of division along the plane GHKL, it will be
seen that each spheroid would have to detach itself from four of the
neighbouring layer, two of which touch it on the flattened surfaces,
and two at the edges. So that this division is likewise more difficult
than that which is made parallel to one of the surfaces of the
crystal; where, as we have said, each spheroid is detached from only
three of the neighbouring layer: of which three there is one only
which touches it on the flattened surface, and the other two at the
edges only.

However, that which has made me know that in the crystal there are
layers in this last fashion, is that in a piece weighing half a pound
which I possess, one sees that it is split along its length, as is the
above-mentioned prism by the plane GHKL; as appears by colours of the
Iris extending throughout this whole plane although the two pieces
still hold together. All this proves then that the composition of the
crystal is such as we have stated. To which I again add this
experiment; that if one passes a knife scraping along any one of the
natural surfaces, and downwards as it were from the equilateral obtuse
angle, that is to say from the apex of the pyramid, one finds it quite
hard; but by scraping in the opposite sense an incision is easily
made. This follows manifestly from the situation of the small
spheroids; over which, in the first manner, the knife glides; but in
the other manner it seizes them from beneath almost as if they were
the scales of a fish.

I will not undertake to say anything touching the way in which so many
corpuscles all equal and similar are generated, nor how they are set
in such beautiful order; whether they are formed first and then
assembled, or whether they arrange themselves thus in coming into
being and as fast as they are produced, which seems to me more
probable. To develop truths so recondite there would be needed a
knowledge of nature much greater than that which we have. I will add
only that these little spheroids could well contribute to form the
spheroids of the waves of light, here above supposed, these as well as
those being similarly situated, and with their axes parallel.

_Calculations which have been supposed in this Chapter_.

Mr. Bartholinus, in his treatise of this Crystal, puts at 101 degrees
the obtuse angles of the faces, which I have stated to be 101 degr