neous assemblage of double, or triple or multiple Boscovich atoms. On the contrary, any arbitrarily chosen values may be given to the bulkmodulus and to the rigidity, by proper adjustment of the law of force, even though we take nothing more complex than the homogeneous assemblage of double Boscovich atoms above described.

The most interesting and important part of the subject, the kinetic, was, for want of time, but slightly touched in the communication to Section A. The author hopes to enter on it more fully in a future communication to the Royal Society of Edinburgh.

THE MODERN THEORY OF LIGHT.*

By Prof. OLIVER J. LODGE.

To persons occupied in other branches of learning, and not directly engaged in the study of physical science, some rumor must probably have travelled of the stir and activity manifest at the present time among the votaries of that department of knowledge.

It may serve a useful purpose if I try and explain to outsiders what this stir is mainly about and why it exists. There is a proximate and there is an ultimate cause. The proximate cause is certain experiments exhibiting in a marked and easily recognizable way the already theoretically predicted connection between electricity and light. The ultimate cause is that we begin to feel inklings and foretastes of theories, wider than that of gravitation, more fundamental than any theories which have yet been advanced; theories which, if successfully worked out, will carry the banner of physical science far into the dark continent of metaphysics, and will illuminate with a clear philosophy much that at present is only dimly guessed. More explicitly, we begin to perceive chinks of insight into the natures of electricity, of æther, of elasticity, and even of matter itself. We begin to have a kinetic theory of the physical universe.

We are living, not in a Newtonian, but at the beginning of a perhaps still greater, Thomsonian era. Greater not because any one man is probably greater than Newton, but because of the stupendousness of the problems now waiting to be solved. There are a dozen men of great magnitude, either now living or but recently deceased, to whom what we now know towards these generalizations is in some measure due, and the epoch of complete development may hardly be seen by those now alive. It is proverbially rash to attempt prediction, but it seems to me that it may well take a period of fifty years for these great strides to be fully accomplished. If it does, and if progress goes on at

Being the general substance of a lecture to the Ashmolean Society in the University of Oxford, on Monday, June 3, 1889. (University College Magazine, Liverpool, July, 1889, vol. IV, pp. 90-99.)

+ Though indeed a century hence it may be premature to offer an opinion on such a point.

anything like its present rate, the aspect of physical science bequeathed to the latter half of the twentieth century will indeed excite admiration, and when the populace are sufficiently educated to appreciate it, will form a worthy theme for poetry, for oratorios, and for great works of art.

To attempt to give any idea of the drift of progress in all the directions which I have hastily mentioned, to attempt to explain the beginnings of the theories of elasticity and of matter, would take too long, and might only result in confusion. I will limit myself chiefly to giving some notion of what we have gained in knowledge concerning electricity, æther, and light. Even that is far too much; I find I must confine myself principally to light, and only treat of the others as incidental to that.

For now well-nigh a century we have had a wave theory of light; and a wave theory of light is quite certainly true. It is directly demonstrable that light consists of waves of some kind or other, and that these waves travel at a certain well-known velocity, seven times the circumference of the earth per second, taking eight minutes on the journey from the sun to the earth. This propagation in time of an undulatory disturbance necessarily involves a medium. If waves setting out from the sun exist in space eight minutes before striking our eyes, there must necessarily be in space some medium in which they exist and which conveys them. Waves we can not have unless they be waves in something.

No ordinary medium is competent to transmit waves at anything like the speed of light, hence the luminiferous medium must be a special kind of substance, and it is called the æther. The luminiferous æther it used to be called, because the conveyance of light was all it was then known to be capable of; but now that it is known to do a variety of other things also, the qualifying adjective may be dropped.

Wave motion in æther, light certainly is; but what does one mean by the term wave? The popular notion is, I suppose, of something heav ing up and down, or perhaps of something breaking on the shore in which it is possible to bathe. But if you ask a mathematician what he means by a wave, he will probably reply that the simplest wave is

y= a sin (pt-nx)

and he might possibly refuse to give any other answer.

And in refusing to give any other answer than this, or its equivalent in ordinary words, he is entirely justified; that is what is meant by the term wave, and nothing less general would be all-inclusive.

Translated into ordinary English the phrase signifies "a disturbance periodic both in space and time." Anything thus doubly periodic is a wave; and all waves, whether in air as sound waves, or in æther as light waves, or on the surface of water as ocean waves, are comprehended in the definition.

Elas

What properties are essential to a medium capable of transmitting wave motion? Roughly we may say two: elasticity and inertia. ticity in some form, or some equivalent of it, in order to be able to store up energy and effect recoil; inertia, in order to enable the disturbed substance to over-shoot the mark and oscillate beyond its place of equilibrium to and fro. Any medium possessing these two properties can transmit waves, and unless a medium possesses these properties in some form or other, or some equivalent for them, it may be said with moderate security to be incompetent to transmit waves. But if we make this latter statement one must be prepared to extend to the terms elasticity and inertia their very largest and broadest signification, so as to include any possible kind of restoring force, and any possible kind of persistence of motion respectively.

These matters may be illustrated in many ways, but perhaps a simple loaded lath or spring in a vise will serve well enough. Pull aside one end, and its elasticity tends to make it recoil; let it go and its inertia causes it to over-shoot its normal position; both causes together cause it to swing to and fro till its energy is exhausted. A regular series of such springs at equal intervals in space, set going at regular intervals of time one after the other, gives you at once a wave motion and appearance which the most casual observer must recognize as such. A series of pendulums will do just as well. Any wave-transmitting medium must similarly possess some form of elasticity and of inertia.

But now proceed to ask what is this æther which in the case of light is thus vibrating? What corresponds to the elastic displacment and recoil of the spring or pendulum? What corresponds to the inertia whereby it over-shoots its mark? Do we know these properties in the æther in any other way?

The answer, given first by Clerk Maxwell, and now reiterated and insisted on by experiments performed in every important laboratory in the world, is:

The elastic displacement corresponds to electro-static charge (roughly speaking, to electricity).

The inertia corresponds to magnetism.

This is the basis of the modern electro-magnetic theory of light. Now let me illustrate electrically how this can be.

The old and familiar operation of charging a Leyden jar-the storing up of energy in a strained di-electric-any electro-static charging whatever-is quite analogous to the drawing aside of our flexible spring. It is making use of the elasticity of the æther to produce a tendency to recoil. Letting go the spring is analogous to permitting a discharge of the jar-permitting the strained di-electric to recover itself—the elec tro-static disturbance to subside.

In nearly all the experiments of electro-statics ætherial elasticity is manifest.

Next consider inertia. How would one illustrate the fact that water