THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FOURTH SERIES.] 



OCTOBER 1864. 



XXIX. On the Photographic Use of a Silvered- Glass Reflecting 

 Telescope. By Henry Draper, M.D., Professor of Natural 

 Science in the University of New York*. 



SINCE the introduction of mirrors of silvered glass for astro- 

 nomical uses by Eoucault and Steinheil, they have conti- 

 nually increased in favour, and have now assumed very consider- 

 able importance. As a testimonial to their efficiency, I may state 

 that one of 15i inches aperture has furnished me the means of 

 producing photographs of the moon 50 inches in diameter, well 

 defined, and of good general effect. 



My attention was first directed to them in 1860, owing to 

 some remarks made by Sir J. E. W. Herschel to my father. I 

 had at that time a reflector of speculum-metal of 15 inches aper- 

 ture and 12 feet focal length, mounted in an observatory which 

 was described in a paper read at the Oxford Meeting of the 

 British Association in 1860. Soon afterwards the speculum 

 was replaced by a silvered-glass mirror of 15 ~ inches aperture 

 and 12J feet focal length, which I had ground, polished, and 

 silvered. Since then more than a hundred mirrors have been 

 prepared in my workshop, in order to secure two of the highest 

 perfection. The full account of these operations, illustrated by 

 forty-seven woodcuts, has just been published by the Smithsonian 

 Institution at Washington, in the ' Contributions to Science/ 



In the past four years many opportunities have presented 

 themselves for learning the qualities of these instruments, what 

 their defects and advantages are, and the best means of con- 

 structing them. When the eye has become experienced in 



* Communicated by the Author. 

 Phil. Mag. S. 4. Vol. 28. No. 189. Oct. 1864. S 



