Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/640

 618 LIGHTHOUSE FIG. 4. Courses of various Lighthouse Towers. II. LIGHTHOUSE ILLUMINATION. What is required of every lighthouse apparatus is either the equal distribution of the rays constantly or periodically over the whole horizon, or else their unequal distribution over certain azimuths only. The first of these two cases, viz., the equal distribution of the light, will be best understood by explaining the different manner in which the rays are operated on by the apparatus for a fixed light and by that for a revolving light. The characteristic of a jfcmtf light, which is that of being seen constantly and always of the same power round the whole horizon, might no doubt be perfectly produced by a naked flame without any apparatus, but then all the rays which did not fall on the navigable track of shipping would be lost to the sailor. In order then to intercept and utilize those rays which, instead of falling on this navigable track, would either go upwards to the sky or downwards on the shore close to the lighthouse tower, and on that part of the sea which is very near the shore, we must have recourse to optical agents both for bending down the rays which naturally point too high, and for bending up those which point too low. It thus appears that the apparatus for a fixed light should bend the rays in the vertical plane only, but should not interfere with their natural horizontal divergence in azimuth. The demands which are made on a light that has to revolve are not nearly so great as on one that is fixed, for the revolving light does not, like the fixed, require to illuminate the whole horizon simultaneously, but only each point of it at successive intervals of time. When the dark intervals occur, the rays from the flame which are then pointing in the direction of the dark spaces should therefore have their directions so altered laterally as to pass into the adjoining light spaces and thus to increase the power of the luminous flashes. A revolving light, though supplied by a flame of the same power as a fixed, will thus necessarily be far more intense, as it does not lose its power by diffusing the rays constantly over the whole horizon, but gathers them up into a number of separate bundles or beams of great intensity. The apparatus of a revolving light has consequently more optical work to do than that of a fixed, for the rays must be bent not only in the vertical plane but laterally in the horizontal and in all intermediate planes as well. In the construction of lighthouse apparatus either metallic or glass agents may be employed, but it has been found by experiments that a great saving of light (about 25 per cent.) is effected when glass only is used. All kinds of apparatus may conveniently be ranged either under the catoptric system, where metallic reflexion only is used, the dioptric where the material employed is wholly glass producing refraction and total reflexion, or the catadioptric, in which both glass and metal are employed. CATOPTRIC SYSTEM of Illuminating every Azimuth with Light of equal Power cither Constantly or Periodically. Passing over the early and rude expedients of such night marks as open coal fires or naked candles placed in glazed lanterns, we shall confine our attention to the gradual development of those optical designs which are now or were lately adopted for lighthouse ap paratus. Parabolic Reflec tors. In 1763, or at latest before 1777, parabolic re flectors were first used for lighthouse illumination by Mr Fig. 5. Fig. 6. Hutchinson, dockmaster of Liverpool. In his work on Practica* Seamanship, published in 1777, he states that the Mersey lights were fitted with reflectors (figs. 5 and 6) formed of small facets of silvered Catoptr system