cghf

Updated: May 24, 2025

And this arc AF is the measure of the angle ACF in the figure of the crystal. 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.

Christiaan Huygens - Treatise on Light