The application of lenses to lighthouses seems to have been proposed in England, and essayed at the South Foreland, as far back as 1752; but owing to mechanical imperfection, they were found to give a light inferior to that of the paraboloidal reflectors, and consequently were abandoned. Buffon, the great naturalist, suggested that a lens might be constructed in concentric zones out of a solid piece of glass; but the difficulties of the process have proved too great to be overcome. In, or about 1773, Condorcet proposed that burning lenses should be built up in separate pieces; and a similar method was described by Sir David Brewster in 1811. The same construction was quite independently discovered by the ingenious Fresnel in 1819; and soon afterwards he constructed a lens, placed a powerful lamp in its focus, and rendered it available for the practical purposes of a lighthouse. He is therefore the author, if not the inventor, of the highly successful system of illumination which bears his name.
But before entering into a minuter description of the work, let us learn a few particulars of the man.
ANNULAR BUILT LENS.
Jean Augustin Fresnel was born at Broglie, near Bernay, in the French department of the Eure, on the 10th of May 1788. When eight years old the future savant was still ignorant of his letters; a fact, says one of his biographers, to be attributed not so much to his delicate constitution as to a deep-rooted dislike for the study of languages, and, in general, for all exercises dependent upon the memory. But, on the other hand, at nine years of age, he was already distinguished by the experimental researches he had made in the domain of physics; which induced his parents to send him to the Polytechnic School. Here, rising step by step with remarkable rapidity, he eventually became Engineer of Ports et Chaussèes.
In 1819 he carried off the prize proposed by the Academy of Sciences on the difficult question of the diffraction of light. His investigations had long been directed to optical subjects, and hence, when the French Government established the Lighthouse Commission, Arago, who was nominated president, immediately appointed Fresnel to the important post of secretary.
Fresnel recognized the peculiar advantages of a plano-convex lens to refract in lines nearly parallel to their axis all the rays emanating from their foci. Like Condorcet and Brewster, who, as we have seen, had also turned their attention to the problem, though only for so far as concerned burning instruments, he asked himself whether, by arranging the lenses in stages, it was not possible to correct their spherical aberration—a defect which becomes all the more signal as the size of the lenses is enlarged—and, consequently, to obtain full command over the rays of a lamp.