The electric light, as yet, is applied only to lighthouses with fixed lights, for a special arrangement would be necessary in the lenticular apparatus before it could be employed with the same advantage in the production of intermittent lights (_feux à éclipses_). Experiments, however, have been made in this direction, which promise good results. Yet, in the present state of its mechanical conditions, the system of electric illumination does not seem susceptible of any very great development upon our shores. It cannot be applied economically to lights which require no very great intensity,—and these lights are the most numerous; and, on the other hand, the intricate constructions which it necessitates, the chances of accident which it presents, and the quantity of coal which it consumes, are obstacles to its employment in lighthouses isolated at sea, whose communications with the mainland are liable to interruption, and where it is of importance to reduce as much as possible the dimensions of the edifice as well as the amount of transport. However this may be, the electric light would seem destined to render valuable services to navigation at every point where it can be employed, and like the two great inventions which the history of marine lightage signalizes—that of paraboloidal reflectors, and next, that of the lenticular apparatus—it constitutes a special and noteworthy progress, under the threefold aspect of intensity of light, diversity of character, and the value of luminous unity. We may add that Mr. Wilde, of Manchester, has invented a powerful electro-magnetic apparatus for lighthouse illumination, which may probably prove valuable. Modifications of the lime light, resulting from the action of an oxy-hydrogen flame upon a surface of prepared lime, have also been suggested; and the least powerful of these surpasses in brilliancy the best oil-lamp, as that surpasses the open coal-fire. We may, therefore, expect that as the latter barbarous mode of illumination gave way to the catoptric, or _reflecting_ system, so will the dioptric, before many years have passed, succumb to some ingenious apparatus capable of utilizing either the lime or the electric light. * * * * * We now return to Fresnel’s system, the _dioptric_, which is pretty generally adopted in the British lighthouses. We must here premise that the system is based upon the laws of the refraction of light. But, says the non-scientific reader, what do you mean by the _refraction_ of light? I know very well what reflection is; I am not so clear as to refraction.