DISCUSSION. ON LIGHTHOUSE APPARATUS AND LANTERNS
Minutes of the Proceedings of the Institution of Civil Engineers
Mr. HENDERSON said he had been anxious to furnish some detailed particulars as to the construction of lighthouse apparatus and lanterns, inasmuch as the Paper by Mr. Chance treated exclusively of the optical portion of the subject : the two Papers together might, he thought, be regarded as illustrating the best lighthouse practice of the day. He had not brought forward the system of lights constructed of metallic reflectors, because there were many published treatises on that subject, among
... subject, among which might be mentioned the one by M. LBorxe Reynaud. There were at present four different varieties of lanterns in use in this country, each of which had its own supporters. In arriving at an opinion as to which lantern was the most advantageous, he thought economy of light and money were both elementa of great importance. Mr. REDMAN directed attention to the fact that the use of lenses for lighthouse purposes was of English origig. The lenses originally employed were of one piece of glass for the entire height, but the thickness of the lens, and the consequent absorption of light were so great, that lenses of that kind were abandoned. Buffon fist proposed the grinding of the lenses in steps, or concentric rings, to overcome this objection ; but the difficulty of shaping a solid piece of glass to this form led to its abandonment. Condorcet, in 1773, suggested the 'building up' of the lens in separate pieces. Sir David Brewster also, in 1811, and Fresnel, in 1822, proposed the same method, and the latter first applied it practically, and it was generally assumed without previous knowledge of what had been done by his predecessors. As far back as the year 1827 experiments were made by the Trinity House with Sir David Brewster's lenses, which were abandoned ; SO that in reality the present system of French lights was, to a great extent, of English origin. He might add that lenses were first used on the south coast of England sixty years or seventy years back. Mr. WILFRID AIRY remarked that, according to the proportions given by the Author, the glam, as made by the Frepch and the English manufacturers, Mered materially in quality. In the French article there was a preponderance of lime over S & , while in the English glass the soda was in excess 'of the lime. He could not but think tbat a very Merent glass must result from such dissimilar admixtures, and he would ask whether, in addition to the difference in the refractive index, the one was not harder than the other, and also unlike in colour? It was stated that a margin of inch was allowed round the lens as it was cast. As the grinding of glass was a laborious process, he thought it would be desirable, if possible, to reduce that margin, unless it was necessary, from the purity of the glass being injured tQ that depth from contact with the mould, or in consequence of inaccuracy in the casting, so as to allow of the grinding it up to the true shape. D 2 Downloaded by  on [12/09/16].