PRESIDENT’S ADDRESS—SECTION A. 4] 
estimate the rate of breaking down of the field that can occur in 
it. With respect to the. deviation of the calculated index of 
refraction from the true index, as in vegetable oils, it is fair to 
observe that the value of K is measured by steady electric force, 
or at all events for electric forces of periods much greater than 
10-75 seconds. If we had means of estimating the value of K 
for these rapid reversals, it is just, possible that all might turn 
out right. Again, on the other hand, our knowledge of refrac- 
tive index for so-called infinite wave length depends on some 
assumption as to the relation between wave length and index of 
refraction. Now the researches of Langley have lately given 
what may be called an unexpected relation, for these qualities in 
the case of rock-salt, and they at least warn us that any deduc- 
tions based on formule derived from observations in the visible 
or ultra-violet spectrum only must be received with great caution. 
Again, the deviations from the law connecting refractive index 
and specific inductive capacity all seem to be pretty much on 
one side. K is greater than the square root of the refractive 
index. In other words, the velocity calculated on the electro- 
magnetic theory is too small for these exceptional cases. We must 
remember, however, that the velocity given by V = 1/y » K 
is a maximum velocity and only occurs when the medium is 
‘practically unmagnetisable, and the polarisation in the direction 
ot the electric force. If by any peculiar action of matter on 
ether, either of these conditions is not fulfilled, which may well 
be the case, we ought not to be surprised to tind such instances 
ot less velocity as are afforded apparently by the vegetable oils. 
Of course such excuse-making as this would be absurd unless we 
had very real evidence in support of our theory—to use Maxwell’s 
phrase, we may only have the first terms of the theory—and we 
must admit that we are in fact brought round again to our old 
question of the relation between matter and ether. 
Since Maxwell’s time, however, a great deal of work has been 
done, and the result of it all must be regarded as confirming the 
theory in a very remarkable manner. The most important work 
in this direction has been accomplished by Hertz, and we will 
deal in the first instance with it. We shall then be in a better 
position to discuss a variety of facts which allow us to discriminate 
between Maxwell's and more general theories. The first sugges- 
tion as to Leyden jar discharge being oscillatory under some 
circumstances appears to have been made by Henry, in 1842, with 
the object of explaining certain anomalies in the observed magne- 
tisation of needles by the jar discharge. Faraday, also, in 
discussing what happens in an open circuit under induction, 
magines that on the removal of the electromotive force there is a 
surging back of electricity in the wire. In 1847 Helmholtz 
predicted, on theoretical grounds, that a Leyden jar discharge 
through a circuit of small dissipation might be oscillatory in 
