The Collaborative International Dictionary of English v.0.48:

Crookes space \Crookes space\ (kr[oo^]ks). [After Sir William
   Crookes, English chemist, who first described it.] (Physics)
   The dark space within the negative-pole glow at the cathode
   of a vacuum tube, observed only when the pressure is low
   enough to give a striated discharge; -- called also Crookes
   layer.
   [Webster 1913 Suppl.]