The merit of having first suggested the building of lenses in separate pieces, seems to be due to CONDORCET, who, in his _Eloge de Buffon_, published so far back as 1773, enumerates the advantages to be derived from this method. Sir DAVID BREWSTER also described this mode of building lenses in 1811, in the _Edinburgh Encyclopædia_; and in 1822, the late eminent FRESNEL, unacquainted with the suggestions of CONDORCET or the description by Sir DAVID BREWSTER, explained, with many ingenious and interesting details, the same mode of constructing those instruments. To FRESNEL belongs the additional merit of having first followed up his invention, by the construction of a lens and, in conjunction with MM. ARAGO and MATHIEU, of placing a powerful lamp in its focus, and indeed of finally applying it to the practical purposes of a Lighthouse. The great advantages which attend the mode of construction proposed by CONDORCET are,--the ease of execution, by which a more perfect figure may be given to each zone and spherical aberration in a great measure corrected, and the power of forming a lens of larger dimensions than could easily be made from a solid piece. Both BUFFON and CONDORCET, however, chiefly speak of reducing the thickness of the material, and do not seem to have thought of determining the radius and centre of the curvature of the generating arcs of each zone, having contented themselves with simply depressing the spherical surface in separate portions. FRESNEL, on the other hand, determined those centres, which constantly recede from the vertex of the lens in proportion as the zones to which they refer are removed from its centre; and the surfaces of the zones of the annular lens, consequently, are not parts of concentric spheres, as in BUFFON’S lens. It deserves notice, that the first lenses constructed for FRESNEL by M. SOLEIL had their zones polygonal, so that the surfaces were not annular, a form which FRESNEL considered less accommodated to the ordinary resources of the optician. He also, with his habitual penetration, preferred the plano-convex to the double-convex form, as more easily executed.[52] After mature consideration, he finally adopted crown glass, which, notwithstanding its greenish colour, he preferred to flint glass, as being more free from _striæ_. All his calculations were made in reference to an index of refraction of 1·51, which he had verified by repeated experiments, conducted with that patience and accuracy for which, amidst his higher qualities, he was so remarkably distinguished.[53] The instruments have received the name of _annular_ lenses, from the figure of the surface of the zones. [52] The plano-convex lens, with its curved side towards the parallel rays, is also a form producing small spherical aberration, a circumstance which may also have influenced his choice.