The introduction of _colour_ as a source of distinction, is the only means of obtaining a sufficient number of varieties. Yet, in itself, it is an evil of no small magnitude. The effect being produced by interposing coloured media between the burner and the eye of the observer, much light is lost by the absorption of those rays which are retained in order to produce the desired appearance. Experiments have been made with almost every colour; but only red, blue, and green have proved useful, and the two latter merely at such short distances as to unfit them for “sea-lights.” Owing to the depth of tint required to produce a marked effect, the red shades generally absorb about from six-sevenths to five-sevenths of the whole light; a loss so immense as certainly to discourage their adoption whenever it can possibly be avoided. The red glass used in France absorbs only four-sevenths of the light; but then, as might be expected, its colour produces a much less signal distinction to the seaman’s eye. In some of the British lighthouses, the lights are very simply and conveniently coloured, by the use of chimneys of red glass, instead of placing large discs in front of the reflectors. We come now to the _Dioptric_[21] _System of Lights_. The application of lenses to lighthouses seems to have been proposed in England, and essayed at the South Foreland, as far back as 1752; but owing to mechanical imperfection, they were found to give a light inferior to that of the paraboloidal reflectors, and consequently were abandoned. Buffon, the great naturalist, suggested that a lens might be constructed in concentric zones out of a solid piece of glass; but the difficulties of the process have proved too great to be overcome. In, or about 1773, Condorcet proposed that burning lenses should be built up in separate pieces; and a similar method was described by Sir David Brewster in 1811. The same construction was quite independently discovered by the ingenious Fresnel in 1819; and soon afterwards he constructed a lens, placed a powerful lamp in its focus, and rendered it available for the practical purposes of a lighthouse. He is therefore the author, if not the inventor, of the highly successful system of illumination which bears his name. [21] From the Greek δίοπτρον, an optical instrument with tube for looking through. Δίοπτρον is from διἁ, through; and ὅπτομαι, I see. But before entering into a minuter description of the work, let us learn a few particulars of the man. [Illustration: ANNULAR BUILT LENS.]