16 R. THRELFALL. 
crystalline or dull grey form, has its resistance altered by the » 
incidence of light waves and recovers somewhat slowly, and that 
it is sensitive to radiation of somewhat limited range as to wave — 
length. Curiously enough, Professor Lodge has noticed that — 
tubes of filings are sensitive also to waves of a more or less 
definite wave length, but of course on a much larger scale. This 
gives us incidentally a hint as to a possible cause of the light 
sensitiveness of selenium. 
In addition to the loose contact method we now have a large 
number of others, some on the “trigger principle,” in which 
small spark precipitates a greater from a convenient source, some 
involving the use of a galvanometer, some an electrometer, some 
using vacuum tubes, others the heating of wires—a whole host of 
methods whereby the study of electrical waves may be facilitated. 
Among the phenomena discovered by Hertz, none have given 
rise to so much uncertainty as the apparently contradictory 
results obtained by different observers as to the velocity of waves 
concentrated in space by means of wires. These doubts have 
practically been set at rest by investigating the expectedly large 
influence of the walls and floor of the rooms in which experiments 
have been made. The ambiguity from this cause was removed 
before Hertz’sdeath. Another trouble arose as to the question 
whether the waves observed had a wave length depending on 
the natural period of the oscillator, or on that of the receiver. 
These questions have been considerably elucidated by a consider- 
ation of the general conditions of a vibrating system subject to 
damping, whence it appears that a heavily damped oscillator oF 
exciter—or, in other words, one which emits energy very fast— 
can only give one or two waves appreciably affecting the resonator, 
whose “free” period then becomes of chief importance. 
wave length observed in experimental work accordingly depends 
on whether the exciter or receiver is the more heavily damped. 
If we look at the matter from the point of view of the resonator, 
we see that a resonator will respond almost equally well to vibra- 
