INTRODUCTION. 



FOB few papers published by the Institution has there been a more constant 

 demand than for the memoir by Professor Henry Draper, entitled " On the Con- 

 struction of a Silvered Glass Telescope," originally issued forty years ago, in 1864. 



The paper is of remarkable merit as a summary of, and an addition to, the 

 knowledge existing at the time, but during the long interval which has elapsed, 

 progress has been made in various directions and by various hands. 



On the occasion of a new edition of this classic memoir, it was sought to give 

 an account of the latest knowledge on the subject, and I was gratified to be able 

 to obtain from Mr. Ritchey, whose labors in this direction are so well known, an 

 account of the processes which he has employed for making the great mirrors that 

 have been so effective at the Yerkes Observatory, and it has been decided to re- 

 publish, with the original Draper memoir, but as an entirely independent con- 

 tribution to the subject, the present article by Mr. Ritchey. 



The great refracting instruments which have been produced in recent years 

 have not superseded the use of the reflector, which, on the contrary, is occupying a 

 more and more important place. 



The reader is here presented with the most recent methods and results needed 

 in the construction of great mirrors for modern reflecting telescopes. 



S. P. LA.NGLEY, 



Secretary of the Smithsonian Institution. 

 WASHINGTON, June, 1904. 



