. Lighthouse construction and illumination; . eady explained in previousChapters, except to point out that, from its inferiority asregards power, it should not be adopted where revolving orcondensing lights of greater power can be made available. II. Double Lights.—The risk of confusion among fixedlights may be prevented by showing two lights, separated DISTINCTIONS OF LIGHTS. 127 vertically from each other in the same tower, as proposed in1810 for the Isle of May (Fig. 98) by Mr. E. Stevenson, whoafterwards introduced this dis-tinction at Girdleness, Aber-deenshire, in 1833. The effectsof irr
. Lighthouse construction and illumination; . eady explained in previousChapters, except to point out that, from its inferiority asregards power, it should not be adopted where revolving orcondensing lights of greater power can be made available. II. Double Lights.—The risk of confusion among fixedlights may be prevented by showing two lights, separated DISTINCTIONS OF LIGHTS. 127 vertically from each other in the same tower, as proposed in1810 for the Isle of May (Fig. 98) by Mr. E. Stevenson, whoafterwards introduced this dis-tinction at Girdleness, Aber-deenshire, in 1833. The effectsof irradiation tend to blendtogether the visual images ofsuch lights, long before theirdistance apart has become sosmall a fraction of the ob-servers distance from thetower as no longer to sub-tend the angle of the mini-micm visible. The distance atwhich the two lights appearseparate can be found from ithe following table by Stevenson,^ founded on the fact that two lights 6 feetapart are seen just separate at a distance of a nautic Fi?. 98. Distance of the Observer in Nautic Miles. Verticaldistance infeet betweenthe Lights. Distance of the Observer in Nautic Miles. Verticaldistance infeet betweenthe Lights. Distance of the Observer in Nautic Miles. Verticaldistance infeet betweenthe Lights. Distance of the Observer in Nautic Miles. Verticaldistance infeet betweenthe Lights. 12345 6-0212-0418-0624-0830-10 6 7 8 9 10 36-1242-1448-1654-1860-20 1112131415 66-2272-2478-26 84-2890-30 1617181920 96-32102-34108-36114-38l20-40 ^ Rudimentary Treatise on Lighthouses, Part III., 1850. 128 LIGHTHOUSE ILLUMINATION. III. Fixed Light, varied ly Flashes.—The mode of produc-ing this distinction has already been described (Chap. II. 18,30, 7). It is still largely used on the coasts of France, buthas seldom been employed in this country. The defect, whichis a serious one, is the great inequality of power between thefixed light and the revolving flash, for as the former can beseen for
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