The Collaborative International Dictionary of English v.0.48:

Leucoscope \Leu"co*scope\ (l[=u]"k[-o]*sk[=o]p), n. [Leuco- +
   -scope.] (Physics)
   An instrument, devised by Professor Helmholtz, for testing
   the color perception of the eye, or for comparing different
   lights, as to their constituent colors or their relative
   whiteness.
   [1913 Webster]