240 Electrical Insulation in High Vacua. [March, 
are the result of mental associations alone — such illusions, 
in faCt, as those of the windmill and the flying crow. Such 
waves of nervous disturbance have, it would seem, a definite 
rate of propagation, probably not independent of the nature 
of the moving image with respeCt to colour, relative lumin- 
osity, and apparent magnitude. But whether these waves 
of sensible impression are due to a physical motion of any 
structures of the retina I am not yet prepared to offer an 
opinion. 
VI. ON ELECTRICAL INSULATION IN HIGH 
VACUA.* 
By William Crookes, F.R.S. 
S HE following experiments were suggested in the course 
of an investigation on the passage of an induction 
current through highly exhausted tubes. The main 
branch of the research being likely to occupy my attention 
Fig. i. 
for some time, I may be unable to return to these less im- 
portant offshoots. 
A pair of gold leaves were mounted, as for an electroscope, 
in a bulb blown from English lead glass tubing. The leaves 
* A Paper read before the Royal Society, February 20th, 1879. 
