thickness of the Lens , we will here again resume, viz. mpdr ρ - nd ρ t + npr ρ t / mdr + md ρ - mpr ρ - m - ndt + nrt = f .
And let it be required to find the Focus where a whole Sphere will collect the Beams proceeding from an Object at the distance d : Here t is equal to 2 r , and r equal to ρ. And after due Reduction, the Theorem will stand thus, mpdr - 2ndr + 2nprr / 2 nd + 2 nr - mpr = f ; but if d be Infinite, it is contracted to mpr /