. Lighthouse construction and illumination; . ave is of theEddystone, where in 1G 9 6 Winstanley placed tallow candleson a chandelier, also surrounded and protected from the windand rain by a glazed lantern. It may here be noticed, thoughin strictness belonging to the dioptric system, that in 1759a London optician proposed, as Smeaton tells us, to grind theglass of the lantern to a radius of 7^ feet; but the descrip-tion is too vague to admit ofmore than conjecture as to thenature of the apparatus whichhe had in view. The ideawas, however, an important one,inasmuch as it contained thegerm of t
. Lighthouse construction and illumination; . ave is of theEddystone, where in 1G 9 6 Winstanley placed tallow candleson a chandelier, also surrounded and protected from the windand rain by a glazed lantern. It may here be noticed, thoughin strictness belonging to the dioptric system, that in 1759a London optician proposed, as Smeaton tells us, to grind theglass of the lantern to a radius of 7^ feet; but the descrip-tion is too vague to admit ofmore than conjecture as to thenature of the apparatus whichhe had in view. The ideawas, however, an important one,inasmuch as it contained thegerm of the dioptric mode ofillumination. Fresnel states thatin 1759 lenses were actuallyused in some English lighthouses,but were in all probability impro-perly applied, for their use was afterwards abandoned. It wasdoubtless one of these which is sho-svn in Fig. 18 from a draw-ing made at Portland Hghthouse in 1801. The effect of suchan assemblage of fixed lenses would be to throw out narrowbeams of light with arcs of darkness between. It is there-. Fis;. 18. 54 LIGHTHOUSE ILLUMINATION. fore not surprising that siicli a partial mode of illuminationwas discontinued. Catopteic System of Illuminating eveey Azimuth with LIGHT OF equal POWEE, EITHEE CONSTANTLY OE PEEIODICALLY. 5. Parabolic Reflectors.—It was apparently not till 1763that optical principles were for the first time correctly ap-plied to lighthouses. Mr. Hutchinson, dockmaster at Liverpool,states in his book on Practical Seamanship), published in1777, that lighthouses were erected at the Mersey in 1763 ;and, at page 180, that they were fitted with reflectors formedof plates of silvered glass, and made, as he says, as nearlyas they can be to the parabolic curve. Figs. 19 and 20
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