40 PRESIDEN'’S ADDRESS—SECTION A. 
theory is true. In other words, the number expressing the ratio 
of the units and involving length and time as dimensions is the 
same as the velocity of light within the limits of experimental 
error. It is, in fact, almost a question as to whether the velocity 
of light can be got most accurately from direct measurement or 
Pathe comparison of the units. 
Turning now to the velocity of light in media, other than air, 
we can obtain an expression for the velocity in terms of p-and K, 
By the principles of the undulatory theory, and in certain cases 
as the direct result of experiment, we consider that: the velocity 
of light in any medium is inversely as the refractive index of the 
medium. But the velocity on the electromagnetic theory is 
inversely as the square root of the product of the specific induc- 
tive capacity and the permeability. Consequently, if the theory 
is true, the refractive index of any transparent substance should 
be equal to y yp K. Now, for most transparent substances, 
wis nearly the same as it is for air, and consequently the chief 
part of the effect will depend on K. To a first approximation 
we will write: Refractive index = root of specific inductive 
capacity, and see how far this is borne out by experiment. It 
turns out that for some substances, hydrocarbons for instance, 
the equation is true, especially if we take the index by refraction 
for very great wave lengths, but for others the agreement is not 
so good. Again, it is clear that transmission can only take 
place to a sensible extent through insulators—conductors must 
be opaque. What are the facts? The facts are that while it is 
generally true that conductors are opaque and insulators trans- 
parent, it is not always so. Ebonite is an apparently good 
example of an opaque insulator, and most electrolytes are ex- 
amples of transparent conductors. These facts seem at first as if 
they dealt the theory a severe blow, but I think we shall see that 
this is not necessarily the case. Taking J. J. Thomson’s view of 
the way in which conduction goes on, we may suppose that in a 
conductor a certain time has to elapse after the field is estab- 
lished before it is weakened to a certain fraction of its maximum 
value. Now, the waves of light which chiefly affect the eye have 
a period of about 10°7° seconds, consequently if with a given 
electric force it takes longer than this to establish a field and 
break it down, the conductor in which this occurs will behave as 
an insulator for forces of a frequency greater than this. An 
estimate of the frequency which the Slevtrie forces can have may 
be made from the known specific conductivities, and such a 
calculation has actually been made by J. J. Thomson. The 
result seems to me entirely satisfactory, and the apparent dlis- 
crepancy as to the opacity of some insulators and transparency of 
some electrolytes need no longer trouble us. We are still, how- 
ever, left partly in the dark as to the transparency of gold leaf, 
which is possibly greater than it ought to be, even when we 
