Dr. Draper's Telescope. 373 



some/' the doctor says, "which have been used as diagonal 

 reflectors in the Newtonian, and have been exposed during a 

 large part of the day to the heat of the sun concentrated by 

 the 15-^ inch mirror. These small mirrors are never covered, 

 and yet the one now in the telescope has been there a year, 

 and has had the dusty film, like that which accumulates on 

 glass, polished off it a dozen times." 



Besides other interesting optical particulars, the memoir 

 contains many directions for the successful practice of celestial 

 photography, some of which might be found equally valuable 

 for terrestrial purposes ; and to these we may advert on a 

 future occasion ; adopting for the present the author's closing 

 remarks : " In concluding this account of a Silvered Glass 

 Telescope, I may answer an inquiry which, doubtless, will be 

 made by many of my readers, whether this kind of reflector 

 can ever rival in size and efficiency such great metallic specula 

 as those of Sir W. Herschel, the Earl of Eosse, and Mr. Lassell? 

 My experience in the matter, strengthened by the recent suc- 

 cessful attempt of M. Foucault to figure such a surface more 

 than thirty inches in diameter, assures me that not only can the 

 four and six feet telescopes of those astronomers be equalled, 

 but even excelled. It is merely an affair of expense and 

 patience. I hope that the minute details I have given in this 

 paper may lead some one to make the effort." 



