How could we reply, how could our thoughts and glances penetrate the infinite and the invisible, we who do not understand nor even see the thing by which we see and which is the source of all our thoughts? In fact, as has been very justly observed, man does not see light itself. He sees only matter, or rather the small part of the great worlds which he knows by the name of matter, touched by light. He does not perceive the immense rays that cross the heavens save at the moment when they are stopped by an object akin to those with which his eye is familiar upon this earth: were it otherwise, the whole space filled with innumerable suns and boundless forces, instead of being an abyss of absolute darkness, absorbing and extinguishing shafts of light that shoot across it from every side, would be but a monstrous and unbearable ocean of flashes. And, if we do not see the light, at least we think we know a few of its rays or its reflections; but we are absolutely ignorant of that which is unquestionably the essential law of the universe, namely, gravitation. What is that force, the most powerful of all and the least visible, imperceptible to our senses, without form, without colour, without temperature, without substance, without savour and without voice, but so awful that it suspends and moves in space all the worlds which we see and all those which we shall never know? More rapid, more subtle, more incorporeal than thought, it wields such sway over everything that exists, from the infinitely great to the infinitely small, that there is not a grain of sand upon our earth nor a drop of blood in our veins but are penetrated, wrought upon and quickened by it until they act at every moment upon the farthest planet of the last solar system that we struggle to imagine beyond the bounds of our imagination. Shakspeare’s famous lines, “There are more things in heaven and earth, Horatio, Than are dreamt of in your philosophy,” have long since become utterly inadequate. There are no longer more things than our philosophy can dream of or imagine: there is none but things which it cannot dream of, there is nothing but the unimaginable; and, if we do not even see the light, which is the one thing that we believed we saw, it may be said that there is nothing all around us but the invisible.