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mercury, then there was no appearance of a corona, but one 
more resembling what is seen in a Geissler tube, I then let 
the air in gradually, and as the pressure rose the appearance 
changed at first to a most extraordinary mass of bright 
serpents twining and untwining in a knot round the ball, 
then to the branches of an oak tree, and as the pressure 
kept increasing I gradually observed amongst the branches 
a faint corona which I saw at once was what I was looking- 
for, it was formed of pencils of light, forming a light radiating 
envelope round the half diminishing in brightness as it 
receded from the ball, the tree gradually died out until there 
was nothing left but the bright radiating envelope, out of 
which a bright ray would occasionally flash. The diameter 
of this envelope was about three or four times that of the 
ball. It was not steady but flickered so that it would 
appear to turn round ; it consisted of pencils, or, as they are 
termed, bundles of rays, between which there were dark 
gaps. These gaps moved round about the ball; subse- 
quently, however, by sticking sealing-wax on the ball, I 
rendered them definite and permanent. As the pressure of 
air increased, the brush became fitful, and finally ceased 
altogether. It was best when there was about 4 inches of 
mercury in the gauge. By varying the action of the coil I 
could do with different pressures of air, and hence I assume 
that there is a definite relation between the intensity of the 
charge in the ball and the pressure of the air surrounding it 
under which the phenomena can occur. 
The appearance is very faint; so faint that it is difficult 
to see it even when close to the ball, and I find that it takes 
some time before the eye can fully appreciate its beauty. 
It was unfortunately so faint that Mr. Brothers was unable 
