AN ELEMENTARY TREATISE ON OPTICS: PART II. CONTAINING THE HIGHER PROPOSITIONS, WITH THEIR APPLICATIONS TO THE MORE PERFECT FORMS OF INSTRUMENTS. BY RICHARD POTTER, A.M. F.C.P.S. LATE FELLOW OF QUEENS' COLLEGE, CAMBRIDGE; LICENTIATE OF THE ROYAL COLLEGE TAYLOR, WALTON, AND MABERLY, UPPER GOWER STREET, AND IVY LANE, PATERNOSTER ROW; PREFACE. THE modern history of Optical Science may be considered to commence with the discovery of the law of the refraction of light, by Snellius. To Huygens we are indebted for the discussion of the aberration of pencils refracted by spherical surfaces of media, and he applied his investigations to the construction of his double eye-piece, which remains in high estimation to the present day. By good fortune he secured in it the condition required for achromatism, in addition to the advantages of reduced spherical aberration, to which his views were directed, although the conditions for achromatism in eye-pieces were not discovered until long afterwards. Sir Isaac Newton having discovered the unequal refrangibility of the differently coloured rays of the spectrum, shewed the great effect of the chromatic dispersion in producing indistinctness of the optical images produced by lenses. The discovery of the different dispersive powers of different media by Mr. John Dollond, in 1757, and his most successful application of his discovery to the construction of the objectglasses of telescopes, gave a new impetus to the mathematical theory. We find Clairaut,* D'Alembert,† Euler, and Boscovich,|| almost simultaneously investigating the properties of refracted * Mémoires de l'Académie, 1756, 1757, 1762. † Opuscules, Vol. III. 1764. The results of many previous papers condensed in his Treatise, Dioptrica, 1769. || Dissertationes quinque ad Dioptricam pertinentes, 1767, and Opuscules, 1785. |