If I try to make you understand these things partly by old models, and partly by those which we have here, it is only that I may the better be enabled to illustrate that which I look forward to as the higher mode of lighting, by means of the electric lamp and the lime light. [Illustration: Fig. 55.] There is nothing more simple than a candle being set down in a cottage window to guide a husband to his home; but when we want to make a similar guide on a large scale, not merely over a river or over a moor, but over large expanses of sea, how can we then make the signal, using only a candle? I have shewn in this diagram (fig. 55) what we may imagine to be the rays of a candle or any other source of light emanating from the 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 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, downwards, sideways, anywhere within its sphere of action: they are all picked up and sent forward. You thus see what a beautiful and important invention is that of the parabolic reflector for throwing forward the rays of light. [Illustration: Fig. 56.]