vergency

from The Collaborative International Dictionary of English v.0.48

Vergency \Ver"gen*cy\, n.
   1. The act of verging or approaching; tendency; approach.
      [R.]
      [1913 Webster]

   2. (Opt.) The reciprocal of the focal distance of a lens,
      used as measure of the divergence or convergence of a
      pencil of rays. [R.] --Humphrey Lloyd.
      [1913 Webster]