And this arc AF is the measure of the angle ACF in the figure of the crystal.
[Illustration] In the same figure, if the plane CGHF cuts the crystal so that it divides the obtuse angles ACB, MHV, in the middle, it is stated, in Article 10, that the angle CFH is 70 degrees 57 minutes.

This again is easily shown in the same spherical triangle ABF, in which it appears that the arc FQ is as many degrees as the angle GCF in the crystal, the supplement of which is the angle CFH.

Now the arc FQ is found to be 109 degrees 3 minutes.

Then its supplement, 70 degrees 57 minutes, is the angle CFH.
It was stated, in Article 26, that the straight line CS, which in the preceding figure is CH, being the axis of the crystal, that is to say being equally inclined to the three sides CA, CB, CF, the angle GCH is 45 degrees 20 minutes.

This is also easily calculated by the same spherical triangle.