The revolving light is produced by the revolution of a three or four sided frame, having large reflectors grouped on each side, with their axes parallel; and as the revolution exhibits once a minute, or once in two minutes, a light gradually increasing to the maximum, and then just as gradually decreasing to total darkness, its effect is remarkably impressive.
The revolving red and white is obtained by the revolution of a frame whose different sides present red and white lights, and exhibit the following succession:—two white lights after one red, or two red lights after one white.
The flashing light is effected in the same manner as the revolving; but, owing to a different construction of the frame, the reflectors on each of the eight sides are arranged with their rims or faces in one vertical plane, and their axes in a line inclined to the perpendicular—a disposition of the mirrors which, together with the greater quickness of the revolution, showing a flash once in five seconds of time, produces an impressive effect, wholly different from that of a revolving light, and presenting the appearance of an alternating rising and sinking illumination. The brightest and darkest periods being but momentary, this light is also characterized by a rapid succession of bright flashes; whence its name.
REVOLVING APPARATUS ON THE CATOPTRIC PRINCIPLE.
The intermittent light is distinguished by bursting suddenly into view and continuing steady for a short time, after which it is suddenly eclipsed for half a minute. This is due to the perpendicular motion of circular shades in front of the reflectors, by which the light is alternately revealed and hidden.
The double lights (“which are seldom used except where exists a necessity for a leading line, as a guide for taking some channel or avoiding some danger”) are generally exhibited from two towers, one of which is higher than the other. At the Calf of Man, says Mr. Stevenson,[20] a striking variety has been introduced into the character of leading lights, by substituting for two fixed lights, two lights which revolve in the same periods, and exhibit their flashes at the same instant; and these lights are, of course, susceptible of the other variety enumerated above, that of the revolving red and white lights, or flashing lights, coming into view at equal intervals of time. The utility of all these distinctions is to be estimated with reference to their property of at once striking the eye of an observer, and being instantaneously obvious to strangers.
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.