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. * * * * * Let us now transport ourselves to the upper story of a lighthouse, and putting aside the motive mechanism of the apparatus, let us penetrate into the lantern. Cast your glance upon the interior of that immense diamond which we call a _dioptric apparatus_. The first object which strikes our attention is the lamp. As the fire which shone on the summit of the edifice was the soul of the pharos, so the lamp is the soul of the modern lighthouse. It was to this lamp Teulère first directed his attention, when he brought the catoptric system to perfection; and it was to this lamp that Arago and Fresnel addressed themselves when engaged, in their time, in improving the work of Teulère, Argand, and Borda. Only, every lighthouse does not employ the same kind of lamp. In one, we meet with the Carcel lamp, where the oil is elevated to the wick by a clock-work mechanism. In another, it is the Moderator, in which the same function is discharged by a heavy weight surrounding a roller. In others, whose range is limited, it is the Permanent-level lamp, where the reservoir of oil is placed by the side and on the level of the burner, which possesses the power of regulating the supply.