RESEMBLING THE SOLAR CORONA. 205 



purpose. In order to produce the appearance, a certain 

 relation is necessary between the pressure of the air and 

 the intensity of the discharge. It is produced best when 

 the receiver is a glass globe insulated on a glass stand, the 

 ball being supported in its middle by a rod (coated with 

 india-rubber, to prevent any discharge except from the ball). 

 It is only negative electricity that is discharged into the 

 globe*. 



There is great difficulty, even when the apparatus is 

 right, in producing the corona. Using a large coil, I just 

 exhausted the receiver till the pressure was equal to half 

 an inch of 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 that of a most 

 extraordinary mass of bright serpents twining and un- 

 twining in a knot round the ball, then to that of the 

 naked 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 consisted of pencils of light, forming a faint radia- 

 ting envelope round the ball, and diminishing in bright- 

 ness as it receded from it. The tree gradually died out 

 until there was nothing left but the bright radiating enve- 

 lope, out of which a brighter 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 at times it appeared to rotate on the ball. 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; subsequently, however, by sticking seal- 



* What becomes of this electricity is not clear ; when a machine is used 

 it probably distributes itself on the inside of the glass, and induces a corre- 

 sponding charge on the outside. When the coil is used it must escape back ; 

 for I have had it going for hours without any variation. 



