sessing those properties, although, we were not in the habit of regarding them as electrical phenomena, and gave them the name of light. If Maxwell's electrical theory was rejected, there was no more reason for accepting his views concerning light. In like manner, if it was affirmed that light is a phenomenon of an elastic nature, his theory of electricity became impossible. But when his theory was studied without prepossession with the ideas that were current, all the parts could be seen to lend one another a mutual support, like the stones of a vault, and the whole resembled a gigantic arch thrown across the unknown, and uniting two known truths.
The difficulty of the theory did not permit it at first to acquire a large number of partisans. But after its inner sense was discerned it was followed out to its ultimate consequences, and then the value of its fundamental hypotheses was tested. Experiments were at first limited to a few propositions, the accessory parts of the theory. I have compared Maxwell's system to an arch traversing an abyss of the unknown. I might add that it was some time before the abutments could be connected. It was thus put in a position where it could support itself, but the span was too wide to permit any new structure to be built upon it. To accomplish that object pillars were needed, rising from the ground, to support the middle of the arch. The demonstration of the possibility of obtaining electrical or magnetic effects directly from light would constitute one of the pillars and confirm the theory; it would have immediately established the electrical part, and indirectly the optical part of it. The completion and symmetry of the structure demanded the building of both the pillars to which we compare these principles, but one was enough to begin with. The construction of the former pillar has not yet been undertaken; but after a multitude of researches a solid base has been found for the second, with sufficiently ample foundations, on which a part of the pillar has been raised. With the co-operation of many workers it will soon reach the top of the arch and afford support to the weight of the edifice which is to be raised upon it.
I have had the privilege of taking part in this portion of the work. To this fact I owe it that I am now laying my ideas before you; and I hope that I may be excused if I try at present to direct all attention to this part of the edifice. I shall unhappily be obliged, for want of time, to omit the labors of a large number of seekers, and shall be unable to show to what extent my experiments had been prepared for by my predecessors, and how near some of them had come to a definite result.
It does not at first seem so difficult to show whether propagation of electrical or magnetic forces is or is not instantaneous; to
