So, many difficulties and outstanding problems melted away as snow before the sun. Indeed, a reviewer in _Nature_ actually compared Maxwell's work to the sun, his only criticism being that there are spots on the sun itself, which, however, "are not visible save to those whose eyes can bear the full glare of the glowing orb." My eyes certainly were not as strong as that. I could not see the spots, but what I could see was that the sun was not entirely unclouded; what sun always was? It was not always easy to grasp Maxwell's ideas, and one feels a want of unity in his book, due to the fact that it faithfully reproduces his gradual transition from old to new ideas. When we read what Maxwell says of Ampère and Faraday, of the former having removed all traces of the scaffolding by which he had built up a perfect demonstration of his law, whereas Faraday "shews us his unsuccessful as well as his successful experiments, and his crude ideas as well as his developed ones," we feel that, great though the difference may be between the _Experimental Researches_ and Maxwell's largely mathematical Treatise, yet the two works were written in the same spirit. In fact, Maxwell repeatedly expresses his indebtedness to Faraday, from whom he had borrowed part of his fundamental ideas, so that, when there is question of Maxwell's theory, we must often think of Faraday also. Maxwell's followers, of whom there were many, in this country and elsewhere, have perfected the theory in its form and extended it by the introduction of new ideas. Think, for instance, of Poynting's beautiful and important theorem on the flow of energy, determined at every point by the electric and the magnetic force existing in the field, a theorem that has produced more clearness perhaps than any other and which is now so essential that we can hardly recall the state in which physics was when we did not know it. Yet, notwithstanding all innovations of this kind, we always speak, and with full justice, of "Maxwell's Theory." We continue to do so now that we have been led to introduce electric charges supposed to exist in the interior of molecules and atoms, by which we have come to the theory of electrons. And when we refer to those wonderfully simple equations in which the fundamental laws of electromagnetism are embodied with a conciseness that could never have been dreamed of before, we call them "Maxwell's Equations." Surely, though Maxwell did not use them in their modern form, no name could be more appropriate, for the general relations which they express are those that were constantly in his mind. Time does not permit me to dwell at length on the verifications of Maxwell's theory, but I should like to make an exception for two of them. Allow me, in the first place, to say some words on the optical properties of metals.