ch is in the plane
GCFH, and which makes with KL an angle of about 6 degrees 40 minutes,
tending from the side of the solid angle C; and on emerging from the
other side of the Crystal it will turn again parallel to JK, along MZ.
And as, in this extraordinary refraction, the point M is seen by the
refracted ray MKI, which I consider as going to the eye at I, it
necessarily follows that the point L, by virtue of the same
refraction, will be seen by the refracted ray LRI, so that LR will be
parallel to MK if the distance from the eye KI is supposed very great.
The point L appears then as being in the straight line IRS; but the
same point appears also, by ordinary refraction, to be in the straight
line IK, hence it is necessarily judged to be double. And similarly if
L be a small hole in a sheet of paper or other substance which is laid
against the Crystal, it will appear when turned towards daylight as if
there were two holes, which will seem the wider apart from one another
the greater the thickness of the Crystal.

10. Again, if one turns the Crystal in such wise that an incident ray
NO, of sunlight, which I suppose to be in the plane continued from
GCFH, makes with GC an angle of 73 degrees and 20 minutes, and is
consequently nearly parallel to the edge CF, which makes with FH an
angle of 70 degrees 57 minutes, according to the calculation which I
shall put at the end, it will divide itself at the point O into two
rays, one of which will continue along OP in a straight line with NO,
and will similarly pass out of the other side of the crystal without
any refraction; but the other will be refracted and will go along OQ.
And it must be noted that it is special to the plane through GCF and
to those which are parallel to it, that all incident rays which are in
one of these planes continue to be in it after they have entered the
Crystal and have become double; for it is quite otherwise for rays in
all other planes which intersect the Crystal, as we shall see
afterwards.

11. I recognized at first by these experiments and by some others that
of the two refractions which the ray suffers in this Crystal, there is
one which follows the ordinary rules; and it is this to which the rays
KL and OQ belong. This is why I have distinguished this ordinary
refraction from the other; and having measured it by exact
observation, I found that its proportion, considered as to the Sines
of the angles which the incident and refracted rays ma