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. [Illustration: 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. [20] Stevenson, “On Lighthouses,” pp. 105-107.