THE IDENTITY OF LIGHT AND ELECTRICITY. 187 



electric waves, all theorizing becomes superfluous ; the identity 

 of the two orders springs from the experiments themselves. Suc- 

 cess 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 conduct- 

 ors which it meets ; and it becomes sensible when we arm our- 

 selves 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 con- 

 ducting 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 pass- 

 ing 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 cur- 

 rents 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 identifi- 

 cation 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 lim- 

 ited to ethereal undulations of a few fractions of a millimetre, 

 but includes waves the length of which is measured in deci- 

 metres, metres, and kilometres. But, enlarged as it is, it is still 

 only an appendage to electricity. That gains yet more ad van- 



