108 



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 briglit 

 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 baU diminishing in brightness as it 

 receded from the ball, the tree gradually died out until there 

 was nothing left but the briglit radiating envelope, out of 

 which a bright ray would occasionally flash. The dia.meter 

 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 aj^preciate its beauty. 

 It was unfortunately so faint that Mr. Brothers was unable 



