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 take the half rays on the one side and could send them by any contrivance over to the other side, I should gain an advantage in light on the side to which I directed them. This is effected in a beautiful manner by the parabolic mirror, by means of which I gather all that portion of the rays which are included in it; upwards, down-
