ween the three equal plane angles, and imagining in it the three
principal sections, one of which is perpendicular to the face DC and
passes through the edge CF, another perpendicular to the face BF
passing through the edge CA, and the third perpendicular to the face
AF passing through the edge BC; I knew that the refractions of the
incident rays belonging to these three planes were all similar. But
there could be no position of the spheroid which would have the same
relation to these three sections except that in which the axis was
also the axis of the solid angle C. Consequently I saw that the axis
of this angle, that is to say the straight line which traversed the
crystal from the point C with equal inclination to the edges CF, CA,
CB was the line which determined the position of the axis of all the
spheroidal waves which one imagined to originate from some point,
taken within or on the surface of the crystal, since all these
spheroids ought to be alike, and have their axes parallel to one
another.

26. Considering after this the plane of one of these three sections,
namely that through GCF, the angle of which is 109 degrees 3 minutes,
since the angle F was shown above to be 70 degrees 57 minutes; and,
imagining a spheroidal wave about the centre C, I knew, because I have
just explained it, that its axis must be in the same plane, the half
of which axis I have marked CS in the next figure: and seeking by
calculation (which will be given with others at the end of this
discourse) the value of the angle CGS, I found it 45 degrees 20
minutes.

[Illustration]

27. To know from this the form of this spheroid, that is to say the
proportion of the semi-diameters CS, CP, of its elliptical section,
which are perpendicular to one another, I considered that the point M
where the ellipse is touched by the straight line FH, parallel to CG,
ought to be so situated that CM makes with the perpendicular CL an
angle of 6 degrees 40 minutes; since, this being so, this ellipse
satisfies what has been said about the refraction of the ray
perpendicular to the surface CG, which is inclined to the
perpendicular CL by the same angle. This, then, being thus disposed,
and taking CM at 100,000 parts, I found by the calculation which will
be given at the end, the semi-major diameter CP to be 105,032, and the
semi-axis CS to be 93,410, the ratio of which numbers is very nearly 9
to 8; so that the spheroid was of the kind which resembles a
com