centre of a sphere in all directions round to infinite distances. After this simple kind of light had been used for some time—it being found to be liable to be obscured by fogs, or distance, or other circumstance—there arose the attempt to make larger lights by means of fires; and after that there was introduced a very important refinement in the mode of dealing with the light, namely, the principle of reflection,—for, understand this (which is not known by all, and not known by many who should know it), that when we take a source of light—a single candle, for instance, giving off any quantity of light—we can by no means increase that light: we can make arrangements around and about the light, as you see here, but we can by no means increase the quantity of light. The utmost I can do is to direct the light which the lamp gives me by taking a certain portion of the rays going off on one side and reflecting them on to the course of the rays which issue in the opposite direction. First of all, let us consider how we may gather in the rays of light which pass off from this candle. You will easily see that if I could
