was received as proved, because the essential facts at least of the new departments were deducible from its premises.
The life of the undulatory theory as a mechanical hypothesis has, however, been a limited one, for it has been borne to the grave in our own time without display, and been replaced by the electromagnetic theory. The cause of its death is shown very plainly when we dissect the corpse. It was carried down by its mechanical constituents. The hypothetical ether, on which the task of undulating was imposed, had to do this under peculiarly hard conditions, for the phenomena of polarization demanded peremptorily that the undulations should be transversal; but such undulations presuppose a rigid body, and Lord Kelvin's calculations have shown, as a final result, that a medium with such properties as this ether must have is not stable; whence the conclusion is inevitable that it can have no physical existence. In order to spare the now accepted electro-magnetic theory of light from such a fate, the lamented Hertz, to whom this theory owes so much, expressly denied that he saw anything else in it than a system of six differential equations. This termination of the evolution speaks more impressively than I ever can against the permanent usefulness of the formerly current mechanical theorizing.
But I hear it said. Those theories have been so fertile. Yes, they have been so through the sum of correct constituents in them, as they have been damaging through their false ones. What were the correct and what the false had to be determined by long and costly experiments. The result of our discussion so far is first a pure negative. We have learned how not to do it, and it seems to be of little use to follow out such negative results. Yet we can point out a gain here, which will not appear worthless to many of you. We find it possible, as we go, to refute critically a view which had no small credit in its time, and caused great concern to many of those interested in the discussion. I refer to the widely known propositions which the eminent physiologist of the University of Berlin, Emil Du Bois-Reymond, made, first twenty-three years ago, on the occasion of the meeting of naturalists at Leipsic, and afterward in some more widely read writings, relative to the prospects of our future knowledge of nature, and which culminated in the famous expression ignorabimus. In the long controversy which followed this address, Du Bois-Reymond, so far as I can see, remained essentially the victor against all attacks, for all his antagonists proceeded from the same principle from which he inferred his ignorabimus, and his conclusions stood as firmly as that principle. This principle, which in the meanwhile had never been brought in question by any one, was the mechanical theory of the world—the supposition that the solu-
