SBA EVENING DISCOURSES. 
His theory shows why an ordinary alternating dynamo does not radiate appreciably ; 
it does radiate, like every alternating current, but if its frequency is comparable to 
100 a second the amount of energy lost is next to nothing. To get at anything like 
efficient radiation you must have an alternating current of a million a second or more; 
and if you could only work up the oscillations till they were five hundred-million- 
million a second (which sounds preposterous) then you would have the means of 
detecting them. They would be sufficiently rapid then to excite our sense-organ, 
the eye, and give us the sensation of a strong yellow light; for what we call light is 
just an ethereal vibration excited by an electric oscillation of this extravagant 
frequency. 
Still we didn’t know how to produce these oscillations, and still less how to detect 
them. FitzGerald virtually said that the oscillations were there whenever a Leyden 
jar discharged. On what ground was he able to make that assertion ? How did he 
know that an electrical oscillation would generate ether waves just as a tuning-fork 
generates sound waves? He only knew that on the strength of the work of a great 
genius, James Clerk Maxwell, who in 1865 communicated papers to the Royal Society, 
and to the British Association a year or two later, giving the result of his mathematical 
theory of Faraday’s views on electromagnetic phenomena. Maxwell’s equations 
expressing electric and magnetic relations were, and still are, of the utmost importance. 
They are not expressed in the simplest possible form, but they are remarkably complete. 
Simplification came later. But as a foundation for all the work that followed during 
the century, Maxwell’s equations are the basis, and shine with undiminished brightness 
down to the present day. 
This leads me to make a digression on the work and methods of mathematical 
physicists. Their plan in studying any phenomenon is to bethink themselves of what 
is the fundamental fact or process underlying it. They express that process in what 
to them is the simple form of an equation, and -having written down equations 
appropriate to each aspect of the phenomenon, they proceed to combine these equations 
according to certain rules, the rules of pure mathematics, and deduce the consequences. 
It is not a process that comes naturally to ordinary people; indeed, they find a 
difficulty in following it. When they do follow it, they are apt to be lost in admiration, 
first for the insight which enabled them to express the fundamental laws in that 
tractable form, and next for the skill with which the forms have been manipulated, 
so that results could be interpreted which might subsequently be put to the test of 
experiment and thus verified. Verification is always necessary because, though the 
theory may be accurate as far as it goes, it never goes all the way, and it may fail 
in not going far enough. A complete theory of any phenomenon would have to take 
all the universe into account, but no one aims at such a complete theory: they take 
the most essential features of what is happening and ignore the rest. It takes some 
genius to perceive what the most essential features are, and to judge whether the 
other things mayebe ignored or not. When the theory fails to be verified in practice 
it means either that some error has been made in the calculation, or, more probably, 
that something has been ignored which ought not to have been ignored. Thus, for 
instance, to take a trivial example. 
Prof. Tait, the great mathematician of Edinburgh, calculated the trajectory of a 
golf ball, taking into account a good number of the causes governing it—the impact 
of the head of the club, the inertia and elasticity of the ball, the resistance of the air, 
the force of gravity, and so on. His first theory gave him a maximum range which 
no one, however skilled, could hope to exceed. Then, as is well known, his son, 
Lieutenant Tait, a skilled golfer, exceeded it. Whatdid Taitdo? He didn’t abandon 
the theory ; he perceived that something more must be taken into account. What 
he had ignored as unimportant was the spin on the ball. A ball, when struck except 
in a line exactly through its centre, will not only move forward but will be set spinning. 
Everyone knows the effect of spin on a billiard ball. A skilled player purposely puts 
it on by the way he strikes the ball; it will then rebound from the cushion or from 
another ball in a way different from what it would if it were not spinning. A spinning 
ball might move in a curved line. But then a billiard ball is rolling on a table, a golf 
ball is not. Still, it may be rolling on the air through which it is moving. Tait 
perceived that the spin must not be ignored, but must be fully taken into account. 
He remodelled his theory, writing down some more equations to take the spin energy 
into consideration. He thus made a more complete theory, which led to curious 
results, most of which have now been verified by experiment. The practice and the 
