Since then CI is the refraction of the radius BC, and DI is parallel to CM, VC must be to CD as N to GC, according to what has been demonstrated in Article 31.

But as VC is to CD so is BV to DS.

Let ML be drawn perpendicular to CL.

And because I consider, again, the eyes to be distant above the crystal, BV is deemed equal to the semi-diameter CG; and hence DS will be a third proportional to the lines N and CG: also DP will be deemed equal to CL.

Now CG consisting of 98,778 parts, of which CM contains 100,000, N is taken as 156,962.