electric waves, all theorizing becomes superfluous; the identity of the two orders springs from the experiments themselves. Success in this way also is possible. Let us place the conductor that produces the variation of the electric condition in the focus of a large concave mirror. The electric waves will join, and will come forth from the mirror in the form of a rectilinear beam. We can, it is true, neither see nor touch this beam; but we know it is there, because we can see sparks pass from it to the conductors which it meets; and it becomes sensible when we arm ourselves with our electrical resonator. Its properties are all those of a luminous ray. We can, by turning the mirror, send it into different directions. Studying the path which it follows, we may see that it is propagated in a straight line. If we interpose conducting bodies in its way, they will not let it pass; they cast a shadow, but do. not destroy the ray; they reflect it, and we can follow the reflected beam and satisfy ourselves that it follows the laws of the reflection of light. We can also refract it as we do light; and, as we use a prism to study the refraction of light, so we do here. But the dimensions of the waves and of the beam force us to take a very voluminous prism. So we select a cheap substance—pitch or asphalt. Finally, we can study on our ray phenomena which we have heretofore observed only in light, those of polarization. If we place a kind of metallic grate in the track of the beam, we can observe our electric resonator emitting sparks or remaining quiescent in obedience to the same geometric laws as govern the variations in the glow of a ray of light in passing through a polarizing apparatus.
In making these experiments we have come into the domain of optics. In describing them we speak no longer of electricity, but use the language of optics. We do not say that the currents pass along the conductors, or that the electricities unite. We see nothing but undulations crossing one another in space, separating, combining, and re-enforcing or weakening one another. Having started from the domain of pure electricity, we have come step by step to purely optical phenomena. The passage is made for henceforth, and the road has become easy. The identification of light and electricity, which science suspected and theory predicted, has been definitely established, made perceptible to our senses and intelligible to the mind. From the heights we have attained, where the two orders of phenomena are blended, we look into the domains of optics and electricity. They seem more vast than we had supposed them to be. Optics is no longer limited to ethereal undulations of a few fractions of a millimetre, but includes waves the length of which is measured in decimetres, metres, and kilometres. But, enlarged as it is, it is still only an appendage to electricity. That gains yet more advan-