A Treatise On The Principles And Practice Of Harbour Engineering

CHANNEL DEMARCATION. 2/1 effecting the same object, the rays entering the prisms being entirely reflected at one of the surfaces. Mirrors behind the light were similarly replaced. Altogether, a very high percentage of the total illumination is utilised. In conséquence of improvements recently effected, the vertical angle of Fresnel lenses now reaches 80 degrees, and the upper and lower prisms have, in many instances, been suppressed. Yet another application of lens concentration is to be found in the Azimuthal Condensing System of Thomas Stevenson, in which holo- photes and vertical prisms are employed to concentrate the light in special horizontal directions. This is exemplified in the apparatus constructed in connection with the Oronsay Lighthouse, where the light, as a leading light, is required to be seen along two intersecting axes of unequal length, in the one case for a distance of 15 miles, and in the other case for a distance of 7 miles. The dark or landward sector embraces an arc of rather more than 180 degrees. A curions and interesting application of ray deflection is to be found at Armish Rock in the Hebrides. The lighthouse is situated on a rock separated from the Island of Lewis by a channel 500 feet in width. It contains no source of illumination itself, but it receives on a mirror a pencil of light rays from a lighthouse on Lewis. This ray is then deflected by prisms to pass onward in the directions required for the purposes of navigation. As at present constructed, a modern lenticular panel consists of a central, circular, plano-convex lens with annular adjuncts, and upper and lower catadioptrical elements. If used for regular flashes, the optical apparatus will not uncommonly be divided into four panels, each comprising a luminous angle of 90 degrees ; but the number of panels may be decreased or augmented at will. Thus there may be six panels of 60 degrees or eight of 45 degrees, and so on. Yet it must be borne in mind that with an increase in the number of panels, there is a corresponding decrease in the intensity of the light. The beam of maximum power is attained by a single panel of about 160 degrees, with a lenticular mirror behind the light capable of reflecting to the focus all rays impinging upon it. Apparatus of this concentration calls for very rapid rotation, such as would be incompatible with the old system of revolution on rollers. The required rotation is actually achieved by supporting the column of the lenticular apparatus in a bath of mercury, which very materially reduces the friction of movement. The biform, triform, and quadriform arrangements of superimposed lights, depending as they do upon the increase in total illumination instead of upon the unit intensity, have more or less ceased to be generally utilised. They are mainly serviceable in misty weather, the separate burners of which they are composed being individually ignitable and extinguishable at will. Range of Light.—The distance penetrable by rays of light varies obviously with the transparency and opacity of the atmosphere. If the medium were a vacuum, the range would be proportional to the square root of the luminous intensity. This relationship, however, cannot be realised