The Collaborative International Dictionary of English v.0.48:

Crookes tube \Crookes" tube`\ (kr??ks" t?b`). (Phys.)
   A vacuum tube in which the exhaustion is carried to a very
   high degree, with the production of a distinct class of
   effects; -- so called from W. Crookes who introduced it.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Crookes tube
    n 1: the original gas-discharge cathode-ray tube