ON THE MODERN REFLECTING TELE- 

 SCOPE, AND THE MAKING AND TEST- 

 ING OF OPTICAL MIRRORS. 



By G. AV. RiTCHEY. 



INTRODUCTION. 



The present paper describes the methods employed by the writer in the optical 

 laboratory of the Yerkes Observatory in making and testing spherical, plane, para- 

 boloidal, and (convex) hj'pei'boloidal miri'ors. On account of tlie very great import- 

 ance of supporting mirrors properly in their cells when in use in the telescope, a 

 chapter is devoted to the description of an efficient support system for large mirrors. 

 Intimately related to this, and equally important, is the subject of the.moiuiting — the 

 mechanical parts — of a modern reflecting telescope; accordinglj', the final chapter 

 is devoted to a consideration of this subject. 



CHAPTER I. 



DISKS OF GLASS FOR OPTICAL MIRRORS. 



No greater mistake could be made than to assume that cheap and poorly 

 annealed disks of glass, or those with large strias or [)oiiring marks, are good enough 

 for mirrors of reflecting telescopes. While I am not prepared to say that optical 

 glass of the finest quality must be used for mirroi-s in order to secure the best 

 attainable results, it is evident that a very high degree of homogeneity and fi-eedom 

 from strain is necessary in order that the figure of mirrors shall not be injuriously 

 afi'ected by changes of temperature. If it were not necessary to consider the ques- 

 tion of cost, I should advise the use of the finest optical (crown) glass always, in 

 order to be as free as possible from risk ; usually considerations of cost would, in 

 the case of large mirrors, make it necessaiy to choose between such an optical disk 

 of a given size and a somewhat larger one of the kind furnished by the St. Gobain 



