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1. When the sun first appears above the horizon and presents itself suddenly to our view, we feel persuaded that there must be some mode of communication between this luminary and ourselves, which informs us of its existence without the necessity of our coming in contact with it. This mode of communication, which thus takes place at a distance, and through the medium of the sight, constitutes what is called light. Bodies, which are capable of exciting it directly, and of thus making themselves known to us, are called self-luminous, as the sun and stars. It would seem, indeed, that all material substances become self-luminous when their temperature is sufficiently raised, and they lose this property by being deprived of their heat. If they receive light, however, from a luminous body, after they have ceased to be self-luminous, they are still capable of sending to us the light thus received, as if it were their own, and in this case they become visible by reflection. In this way we perceive the objects about us while the sun is above the horizon, and all becomes obscure and invisible when this light is withdrawn.
In all cases, when an object transmits to us a sensation of its existence by means of light, this transmission takes place in a right line; for if we place fine threads of silk or metal parallel to each other and in the same plane, a luminous point situated at some distance beyond the threads in the same plane will be eclipsed by them; but if we move it a little out of the direction of this plane, it will become visible. Moreover, if we take two metallic plates perfectly plane and bring them by degrees toOpt.