Once again, there is the paradoxical fact that this outcome of Maxwell's labours contradicts the very foundation on which he had built his theoretical edifice. For his starting-point had been to form a picture of the electro-magnetic field of force to which he could apply certain well-known formulae of mechanics. This he did by comparing the behaviour of the electrical force to the currents of an elastic fluid - that is, of a material substance. It is true that both he and his successors rightly emphasized that such a picture was not in any way meant as an explanation of electricity, but merely as an auxiliary concept in the form of a purely external analogy. Nevertheless, it was in the guise of a material fluid that he thought of this force, and that he could submit it to mathematical calculation. Yet the fact is that from this starting-point the strict logic of mathematics led him to the discovery that electricity is capable of behaviour which makes it appear qualitatively similar to ... light!