Introduction 1 Sauria 5 Chelonia 104 A Synopsis, in which an attempt is made to define more closely the Genera and Species of Reptiles whose remains are described in the preceding pages 115 Index 121 References to the Plates 123 SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE. ----180---- ON THE CONSTRUCTION OF A SILVERED GLASS TELESCOPE, FIFTEEN AND A HALF INCHES IN APERTURE, AND ITS USE IN CELESTIAL PHOTOGRAPHY. BY HENRY DRAPER, M. D., PROFESSOR OF NATURAL SCIENCE IN THE UNIVERSITY OF NEW YORK. [ACCEPTED FOR PUBLICATION, JANUARY, 1864.] COMMISSION TO WHICH THIS PAPER HAS BEEN REFERRED. Prof. WOLCOTT GIBBS. Com. J. M. GILLISS, U. S. N. JOSEPH HENRY, _Secretary S. I._ COLLINS, PRINTER, PHILADELPHIA. CONTENTS. HISTORICAL SKETCH OF THE TELESCOPE. MEMOIR DIVIDED INTO SIX SECTIONS:-- §1. GRINDING AND POLISHING THE MIRRORS 2 1. _Experiments on a metal speculum._ Corrosion by aqua regia; voltaic grinding 2 2. _Silvering glass._ Foucault’s and Cimeg’s processes; details of silvering a mirror; thickness and durability of silver films; their use in daguerreotyping 2 3. _Grinding and polishing glass._ Division of subject 6 _a._ Peculiarities of glass; effects of pressure; effects of heat; oblique mirrors 6 _b._ Emery and rouge; elutriation of emery 10 _c._ Tools of iron, lead, pitch; the gauges; the leaden tool; the iron tool; the pitch polisher 10 _d._ Methods of examination; two tests, eyepiece and opaque screen; appearance of spherical surface; oblate spheroidal surface; hyperbolic surface; irregular surface; details of tests; atmospheric movements; correction for parallel rays by measure; appearances in relief on mirrors 13