lain sheets of metal, concentrated by
parabolic reflectors, refracted by prisms, concentrated by lenses. I
have at the college a large lens of pitch, weighing over three
hundredweight, for concentrating them to a focus. They can be made to
show the phenomenon of interference, and thus have their wave length
accurately measured. They are stopped by all conductors and
transmitted by all insulators. Metals are opaque, but even imperfect
insulators such as wood or stone are strikingly transparent, and waves
may be received in one room from a source in another, the door between
the two being shut.

The real nature of metallic opacity and of transparency has long been
clear in Maxwell's theory of light, and these electrically produced
waves only illustrate and bring home the well known facts. The
experiments of Hertz are in fact the apotheosis of that theory.

Thus, then, in every way Maxwell's 1865 brilliant perception of the
real nature of light is abundantly justified; and for the first time
we have a true theory of light, no longer based upon analogy with
sound, nor upon a hypothetical jelly or elastic solid.

Light is an electro-magnetic disturbance of the ether. Optics is a
branch of electricity. Outstanding problems in optics are being
rapidly solved now that we have the means of definitely exciting light
with a full perception of what we are doing and of the precise mode of
its vibration.

It remains to find out how to shorten down the waves--to hurry up the
vibration until the light becomes visible. Nothing is wanted but
quicker modes of vibrations. Smaller oscillators must be used--very
much smaller--oscillators not much bigger than molecules. In all
probability--one may almost say certainly--ordinary light is the
result of electric oscillation in the molecules of hot bodies, or
sometimes of bodies not hot--as in the phenomenon of phosphorescence.

The direct generation of _visible_ light by electric means, so soon as
we have learnt how to attain the necessary frequency of vibration,
will have most important practical consequences.

Speaking in this university, it is happily quite unnecessary for me to
bespeak interest in a subject by any reference to possible practical
applications. But any practical application of what I have dealt with
this evening is apparently so far distant as to be free from any
sordid gloss of competition and company promotion, and is interesting
in itself as a matter of pure science.