186 THE SEA. 



them from each other. Catoptric lights admit of nine separate distinctions: 1, fixed; 2, 

 revolving- white ; 3, revolving red and white ; 4<, revolving red with two whites ; 5, 

 revolving white with two reds; 0, flashing; 1, intermittent; 8, double fixed lights; D, 

 double revolving white lights. Mr. Stevenson thus defines their distinctive features : 

 " The first exhibits a steady and uniform appearance which is not subject to any change, 

 and the reflectors used for it are of smaller dimensions than those employed in revolving 

 lights. This is necessary in order to permit them to be ranged round the circular frame, 

 with their axes inclined at such an angle as shall enable them to illuminate every point 

 of the horizon. The revolving light is produced by the revolution of a frame with three 

 or four sides, having reflectors of a larger size grouped on each side with their axes 

 parallel, and as the revolution exhibits once in two minutes or once in a minute, as may 

 be required, a light gradually increasing to full strength and in the same gradual manner 

 decreasing to total darkness, its appearance is extremely well marked. The succession of 

 red and white lights is produced by the revolution of a frame whose different sides 

 present red and white lights, and these afford three separate distinctions, namely, alternate 

 red and white, the succession of two white lights after one red, and the succession of two 

 red lights after one white light. The flashing light is produced in the same manner as 

 the revolving light; but, owing to a different construction of the frame, the reflectors on 

 each of 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 revolutions, which shows a flash once in five seconds of 

 time, produces a very striking effect, totally different from that of a revolving light, and 

 presenting the appearance of the flash alternately rising and sinking, the brightest and 

 darkest periods being but momentary ; this light is further characterised by a rapid 

 succession of bright flashes, from which it gets its name. 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. Its striking appearance is produced 

 by the perpendicular motion of circular shades in front of the reflectors, by which the light 

 is alternately hid and displayed. This distinction, as well as that called the flashing light, 

 is peculiar to the Scotch coast. The double lights (which are seldom used except where 

 there is 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 Gulf of Man 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 two 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 valued 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 necessary in order to obtain a sufficient number 

 of distinctions ; but it is in itself an evil of no small magnitude, as the effect is produced 

 by interposing coloured media between the burner and the observer's eye, and much 

 light is thus lost by the absorption of those rays which are held back in order to cause 



