18 R. THRELFALL. 
acoustical principles from its performance. Tesla, with immense 
experimental resources behind him, succeeded in constructing — 
apparatus which, while emitting longer waves, and waves which 
were less damped, repeated the process much oftener, so that — 
yeferring to the analogy again, the bell might be supposed to be — 
kept ringing a considerable fraction of the time. With this radi- — 
ation Tesla was enabled to perform experiments of a quite neW 
order—especially in high vacuum lamps—where an electrostatic 
discharge (in old fashioned language) was caused to converge on 
infusible bodies, such as carbon or carbon silicide. Incidentally — 
it turned out that air ceases to act as an insulator for such high 
frequencies, and this is probably the greatest drawback to the 
utilisation of such radiation for lighting purposes. Within the — 
last two or three years the matter has been taken up by E. 
Wiedemann and Ebert, who find that in a high vacuum the dis- 
charge produced by almost continuous but individually feeble : 
oscillations is capable of exciting the phosphorescence of certain 
substances ina most remarkable manner. As I believe that it 
is legitimate to hope for improved means of illumination in this 
direction, and as I know that this is a subject interesting to most 
people, I will refer to Ebert’s arrangements rather more closely. 
A description of the apparatus employed is given in Wiedemann’s 
Annalen, Vol. ui, fig. 144, and it may be regarded as a simple 
arrangement of a Tesla apparatus for small output combined with — 
an arrangement for securing very persistent oscillation. It may 
be mentioned in passing, that though the energy of an electrically — 
vibrating system is greater, the greater the capacity—still the 
damping is in general greater* also—so that if one wants to 
keep the oscillations going, one must not use too great capacities; 
while on the other hand too small capacities must not be employed, 
or the energy available becomes too small. This is independent 
of the frictional damping or loss in the apparatus itself which — 
depends directly on the resistance. The apparatus on the table is — 
an exact copy of that described by Ebert. It consists of a “spark — 
gap” fed in any suitable manner, (by a Wimshurst machine in this 
