Types of Lights

Group-flashing Lights.—One of the most useful distinctions consists in the grouping of two or more flashes separated by short intervals of darkness, the group being succeeded by a longer flashing). A modification of the system consists in grouping two or more lenses together and filling the remaining angle in azimuth by a reinforcing mirror. A sectional plan of the quadruple-flashing first-order apparatus at Flamborough, Yorkshire, is shown in fig. 11 on page 92.

Hyper-radial Apparatus.—In 1885 Messrs. Barbier of Paris constructed the first hyper-radial apparatus (1,330 mm. focal dis tance) to the design of D. and C. Stevenson. Apparatus of similar focal distance were subsequently established at a number of other lighthouses. That at Manora Point, Karachi, India (1908) is illustrated in fig. The introduction of incandescent oil burners and electric lamps of focal compactness and high intrinsic bright ness has rendered unnecessary the provision of optics of such large dimensions.

Fixed and Flashing Lights.—The use of these lights, which show a fixed beam varied at intervals by more powerful flashes, is undesirable, though a large number were constructed in the earlier years of dioptric illumination and some are still in exis tence. In certain conditions of the atmosphere it is possible for the fixed-light of low power to be entirely obscured while the flashes are visible, thus the true characteristic of the light is vitiated.

Screens and Cuts.—Screens of coloured glass, intended to distinguish the light in particular azimuths, and of sheet iron, when it is desired to "cut off" the light sharply on any angle, should be fixed as far from the centre of the light as possible, in order to reduce commingling, in the first case, and the escape, in the second case, of the light rays due to divergence. These screens are usually attached to the lantern framing.

Divergence.—A dioptric apparatus designed to bend all inci dent rays of light from the light source in a horizontal direction would, if the flame could be a point, have the effect of projecting a band or zone of light (in the case of a fixed apparatus) and a cylinder of light rays (in the case of a flashing light) towards the horizon. Under such conditions the mariner in the near dis tance would receive no light as the rays would pass above the level of his eye and be visible only on the horizon. In practice this does not occur, sufficient natural divergence being produced ordinarily owing to the magnitude of the flame. When the electric arc or an incandescent electric-lamp of small focal diameter is employed it is sometimes necessary to design the prisms so as to produce artificial divergence. The measure of the natural hori zontal divergence for any point of the lens is twice the angle whose tangent is the ratio of the radius of the illuminating source to the distance of the point from the geometrical centre of the lens and for calculating the vertical divergence the vertical dimen sions of the light source, above and below the focal plane, must in turn be substituted for the radius, and the sum of the angles thus obtained is the total divergence. In fixed dioptric-lights there

is, of course, no divergence in the horizontal plane. In revolving lights the horizontal divergence is a matter of considerable im portance, determining as it does the duration of flash, i.e., the length of time the flash is visible to the mariner.

Fcux-Eclairs or Quick-flashing Lights.

One of the most important developments in lighthouse illuminating apparatus was in the direction of reducing the length of flash, initiated by the French lighthouse authorities about 1891, and shortly afterwards followed in other parts of the world. The early de signed by the French engineers and others, had usually a flash of to sec. duration. As a result of experiments carried out in France in 1903-04, sec. is now generally adopted as the minimum duration for white flashing lights. If shorter flashes are used it is found that the reduction in duration is attended by a corresponding, but not proportionate, diminution in effective in tensity. In the case of many electric flashing-lights the duration is of necessity reduced but the greater initial intensity of the flash permits this loss without serious detriment to efficiency. Red or green requires a considerably greater duration than do white flashes. The intervals between the flashes in single-flash ing lights of this character are also small, usually 2 to 5 seconds. In group-flashing lights the intervals between the flashes are about 2 sec. or even less, with periods of 7 to 15 sec. between the eclipse. Thus two, three or more flashes of, say, half-second dura tion or less follow each other at intervals of about 2 seconds and are succeeded by an eclipse of, say, io seconds. In 1874 Dr. John Hopkinson introduced the improvement of arranging the lenses of a dioptric revolving light, together with their panels of reflect ing prisms, asymetrically, setting them at an angle to produce the group-flashing characteristic. The first apparatus of this type constructed were those now in use at Tampico, Mexico (triple flashing), and the Little Basses light-house, Ceylon (double groups, which may be arranged to show any number up to six flashes, as in the older forms of apparatus. The feu-eclair type of apparatus enables a higher intensity of beam to be obtained by reason of the greater ratio of condensation of light, the employ ment of panels of greater angular breadth than those formerly used being possible with a higher rotatory velocity. It has been urged that short flashes are insufficient for taking bearings, but the utility of a light in this respect does not seem to depend so much upon the actual length of the flash as upon its frequent recurrence at short intervals.