The Collaborative International Dictionary of English v.0.48:

Dactylology \Dac`tyl*ol"o*gy\, n. [Gr. da`ktylos finger +
   -logy.]
   The art of communicating ideas by certain movements and
   positions of the fingers; a form of sign language, especially
   the manual alphabets used by the deaf and dumb.
   [1913 Webster +PJC]

   Note: There are two different manual alphabets, the one-hand
         alphabet (which was perfected by Abb['e] de l'Ep['e]e,
         who died in 1789), and the two-hand alphabet. The
         latter was probably based on the manual alphabet
         published by George Dalgarus of Aberdeen, in 1680. See
         Illustration in Appendix.
         [1913 Webster]