one has far more power and capability, generally speaking, than the other; or, just as either the idea of the reflexion of images or that of the reverberation of sounds may be used to represent certain physical forces and conditions, so may the idea of the attractions and repulsions of centres, or that of the disposition of magnetic fluids, or that of lines of force, be applied in the consideration of magnetic phenomena. It is the occasional and more frequent use of the latter which I at present wish to advocate.... When the natural truth, and the conventional representation of it, most closely agree, then are we most advanced in our knowledge. The emission and æther theories present such cases in relation to light. The idea of a fluid or of two fluids is the same for electricity; and there the further idea of a current has been raised, which, indeed, has such hold on the mind as occasionally to embarrass the science as respects the true character of the physical agencies, and may be doing so even now to a degree which we at present little suspect. The same is the case with the idea of a magnetic fluid or fluids, or with the assumption of magnetic centres of action of which the resultants are at the poles. [Sidenote: THE FUNCTIONS OF THE ÆTHER.] How the magnetic force is transferred through bodies or through space we know not--whether the result is merely action at a distance, as in the case of gravity, or by some intermediate agency, as in the cases of light, heat, the electric current, and, as I believe, static electric action. The idea of magnetic fluids, as applied by some, or of magnetic centres of action, does not include that of the latter kind of transmission, but the idea of lines of force does. Nevertheless, because a particular method of representing the forces does not include such a mode of transmission, the latter is not disproved, and that method of representation which harmonises with it may be the most true to nature. The general conclusion of philosophers seems to be that such cases are by far the most numerous. And for my own part, considering the relation of a vacuum to the magnetic force, and the general character of magnetic phenomena external to the magnet, I am more inclined to the notion that in the transmission of the force there is such an action, external to the magnet, than that the effects are merely attraction and repulsion at a distance. _Such an action may be a function of the æther, for it is not at all unlikely that if there be an æther, it should have other uses than simply the conveyance of radiations._[55]