224 Proceedings of the Royal Society of Edinburgh. [Sess. 
round the edge of the plate instead of through it, a considerable horizontal 
displacement of the plate in the direction of its own length may take 
place, as shown in fig. iv. Observed in the dark, the phenomenon is a very 
beautiful one, the rays of the discharge appearing like luminous elastic 
strings attempting, in a contractile effort to straighten out, to pull the 
plate aside — in fact, like luminous Faraday lines of force. The electrifica- 
tion of the plate is slight, and practically indeterminate as regards sign. 
IV. Characteristics of the Brush Discharge . — When a dielectric is 
placed between two electrodes excited to a potential sufficient to produce 
F»q. iv. 
the brush discharge, examination in the dark reveals some interesting 
details. The discharge is seen to be made up, as in ordinary cases, of a 
bundle of luminous threads, each pursuing a quite independent path ; but 
at one part of their course these threads converge, and pass almost wholly 
through one point. This point lies on the dielectric (usually on the 4- 
side), is brilliantly illuminated, and is found to be in every case the site of 
