14 R. THRELFALL. 
than a bare enumeration of them, nor do I claim anything 
except indulgence for the choice I make. I can only deal with 
those matters coming under my immediate notice which seem to 
me individually to be of the most importance, and I have given 
a preference for work done within the last year. 
Commencing with electricity, the road marked out by Hertz 
has been followed by a host of observers who have devoted them- 
selves to the examination of both stationary and progressive 
wave fields. The general results up to last year are contained in 
a work on “ Recent Researches in Electricity and Magnetism,” 
by Professor J. J. Thomson, of Cambridge—a work intended as 
a supplement to Maxwell’s immortal volumes, and forming, in 
fact, a “third volume” worthy of the other two. The pro- 
duction of this work is probably the most important electrical 
event of the last five years. 
In order to take up the story of electrical developments I must 
briefly refer to the state of the subject where it was left by 
Hertz. Maxwell had given sound reasons for believing that 
electric force is a quantity propagated, not instantaneously 
through space, but at a finite though very rapid rate. If a 
conductor, therefore, could be charged, discharged, and charged 
again in an opposite manner each feature of the process could be 
followed at a distance—a sufficient interval of time being allowed 
to elapse for the propagation from the source to the point in 
question. If this process can be repeated regularly, we shall have 
a succession of corresponding states at the point of observation, 
in every essential similar to the succession of states as to vertical 
position passed through by a cork floating on water over which 
a train of waves is passing. If the regular process be repeated 
very rapidly, we have according to Maxwell, a series of electric 
waves—or periodically varying electric conditions—propagated 
outwards with the velocity of light and, in fact, constituting light 
itself. Now this electrical theory of optics, the electro-magnetic 
theory as it is generally called, was found to fit in with observation 
on light, very well indeed on most essential points, but it 
