330 
notice that we are bound to run the risk of neglecting 
a jewel among the chaff. 
These spasmodic observations, however, are not 
exactly discoveries: they were more akin to vague 
intuitions. The first and gigantic step in the real 
discovery was made by Clerk Maxwell, in or about 
1865: and he made it in mathematical form, not in 
experimental actuality, by one of those superhuman 
achievements which are only possible to our greatest 
mathematical physicists. He did not discover either the 
way to generate ether waves, or to detect them ; but 
he did give their laws: he legislated for them before 
they were born. He knew the velocity with which 
they must move, and gave implicitly, though without 
elaboration, the complete theory of their nature. 
Up to his time the nature of light was unknown. 
All the other theories of light had attempted to explain 
it on mechanical principles, like the vibrations of an 
elastic solid. Light was known to consist of transverse 
waves: the wave-length and the frequency of oscilla- 
tion could be determined. But no one knew what 
was oscillating, nor what the mechanism of propaga- 
tion was. With extraordinary genius Fresnel and 
MacCullagh had explained the phenomena of light in 
all detail as regards reflection, refraction, diffraction, 
interference, and polarisation. But the nature of the 
waves was unknown; and the elastic solid theory, 
though fascinating, was felt by those who dived most 
deeply into it to contain some flaw, and to be, strictly 
speaking, unworkable. Light did not seem explicable 
on dynamical principles—the principles which were so 
fruitfully devised by Galileo and Newton for dealing 
with ordinary matter. 
MacCullagh’s theory indeed was not dynamical, and 
in that respect had some advantage. But it was also 
vaguer and less definite on that account; though, 
being thus indefinite and yet enabling results to be 
achieved, it was less liable to be upset and replaced by 
future discovery. ; 
To Clerk Maxwell we owe the epoch-making discovery 
that light was not a mechanical oscillation at all, that 
the ordinary mechanical properties of matter did not 
apply to it, but that it was explicable solely and wholly 
in terms of electricity and magnetism. It is impossible 
to sum up his discovery in a few words ; but roughly 
we may say that the most obvious outcome was : 
(1) That if electric waves could ever be generated 
they would travel with the velocity of light. 
(2) That light was essentially an electromagnetic and 
not a mechanical phenomenon. 
(3) That the refractive index of a substance was 
intimately related to its dielectric coefficient. 
(4) That conductors of electricity must be opaque to 
light. 
Maxwell showed further, though he did not then 
express it in language of this character, that the ether 
had two great and characteristic constants, of value 
utterly unknown to this day, though guessed at by a 
few speculators like myself ;—one of them the electric 
constant of Faraday called K ; the other the magnetic 
constant of Kelvin called 4. It was impossible then, 
and it is impossible now—though it is not likely always 
to remain impossible—to determine the value or even 
the nature of either of these constants. But Maxwell 
did perceive a way of measuring their product ; and he 
NO. 2784, VOL. 111] 
NATURE 

[Marcu 10, 1923 
was the first to measure it. Their product is known ; 
and it is equal—as he showed it must be—to the 
reciprocal of the square of the velocity of light. 
Well now, this great discovery aroused in us young 
physicists the keenest enthusiasm. In the early 
seventies of last century—I think about 1871 or 1872 
—I remember discussing it with the man we all now 
know and honour, J. A. Fleming, who at that time was 
a fellow student with me in Prof. Frankland’s advanced 
chemical laboratory at the brand-new College of Scicnce, 
South Kensington. A year or two later, at Heidelberg, 
I studied Maxwell’s treatise pretty thoroughly, and 
formed the desire to devote my life if possible to the 
production and detection of Maxwell’s electric waves. 
I used to discuss the possibility of producing these 
waves with my great friend, G. F. FitzGerald, whose 
acquaintance I made at the meeting of the British 
Association in Dublin in the year 1878 ; and he wrote 
some mathematical papers discussing the possibility of 
producing such waves experimentally. I myself also 
spoke at the British Association about them, in 1879, 
1880, and again in 1882 at the Royal Dublin Society. 
FitzGerald, as I say, examined mathematically what 
then seemed the abstruse question of electric wave 
production ; and after some hesitation came to the 
conclusion that direct artificial generation of waves 
was really possible on Maxwell’s theory, in spite of 
certain recondite difficulties which at first led him to 
doubt it. (See “Scientific Writings” of FitzGerald, 
edited by Larmor, pp. go-ror.) Indeed one of his 
papers on the subject was originally entitled “ On the 
Impossibility of Originating Wave Disturbances in the 
Ether by Means of Electric Forces.” The prefix “‘ im ” 
was subsequently dropped ; although his first, or 1897, 
paper concluded thus : 
“ However these [displacement currents] may be 
produced, by any. system of fixed or movable con- 
ductors charged in any way, and discharging them- 
selves amongst one another, they will never be so 
distributed as to originate wave-disturbances pro- 
pagated through space outside the system.” 
In 1882 FitzGerald corrected this erroneous conclusion, 
and referred to some early attempts of mine at pro- 
ducing the waves. (“Scientific Writings,” p. 100.) I 
state all this in order to emphasise the difficulty which 
in those early days surrounded the] subject on its 
theoretical as well as on its practical side. 
In 1883, at the Southport meeting of the British 
Association, FitzGerald took a further step and sur- 
mised that one mode of attaining the desired result 
would be by utilising the oscillatory discharge of a 
Leyden jar—the theory of the oscillations of which had 
been worked out, partly by Helmholtz and more fully by 
Lord Kelvin, 30 years before—if only we had the means 
of detecting such waves when they were generated. 
PRODUCTION OF WAVES. 
In 1887 and 1888 I was working at the oscillatory 
discharge of Leyden jars (initially in connexion with 
the phenomena of lightning), and—with the assistance 
of A. P. Chattock—I then found that the waves 
could be not only produced but also detected, and 
the wave-length measured, by getting them to go 
along guiding wires adjusted so as to be of the right 
eT ina eee 
