62 ARTICLES REFERRED TO IX THE SECRETARY'S REPORT. 



DRAPER'S TELESCOPJE 



BY THE REV. T. W . WEBB, A.M., F. R. A. S. 



From the " Intellectual Obscrcr.r," London. 



It is gratifying- to observe that, amidst all the calamities and distresses and 

 confusion of a most unhappy civil war, the studies of peace have not been 

 wholly lost to sight. A remarkable instance of this is afforded by the recent 

 appearance, among the publications of the American Smithsonian Institution, 

 of a very interesting and valuable memoir, " On the Construction of a Silvered 

 Glass Telescope, 15J inches in aperture, and its use in celestial photography, 

 by Henry Draper, M.D., Professor of Natural Science in the University of New 

 York." A copy of this, through the courtesy of the author, being now in my 

 hands, I have thought that some account of its contents might prove interesting, 

 especially at a time, when silvered glass specula are attracting some attention in 

 England, and (unless we are much mistaken) are likely to be more generally 

 known and valued as most important aids to the progress of observation. 



The opening sentence of this memoir requires, however, we venture to think, 

 a little qualification. " The construction of a reflecting telescope capable of 

 showing every celestial object now known," Dr. Draper tells us, " is not a very 

 difficult task." We should have no hesitation in expunging the negative here, 

 unless it were permitted to add, " when study, and labor, and ingenuity, and 

 perseverance have been brought to bear upon it, equal to those displayed by 

 Dr. Draper." His subsequent remark is of more universal application : " The 

 cost of materials is but trifling compared with the result obtained; and I can 

 see no reason why silvered glass instruments should not come into general use 

 among amateurs. The future hopes of astronomy lie in the multitude of ob- 

 servers, and in the concentration of the action of many minds." His first idea 

 was derived from an examination, in 1857, of Lord Rosse's great reflector, and 

 of the machinery by which it was perfected ; and on his return home in the 

 following year, he resolved to construct a similar, though smaller instrument, 

 larger, however, than any in America, and adapted to celestial photography. 

 A metal speculum was first completed, but was split in two during the winter 

 of 1860 by the expansion of a few drops of water that became frozen in the 

 supporting case ; and his attention was then, at Sir John Herschel's suggestion, 

 turned to silvered glass mirrors, as reflecting more than 90 per cent, of incident 

 light, with only £th of the weight of metal. The year 1861 was occupied in 

 •overcoming the difficulties, of grinding and polishing three 15 J-inch disks of 

 glass, as well as a variety of smaller pieces. Three similar mirrors were found 

 almost essential, as two would often be so much alike that a third was necessary 

 to gain a further step in advance. One was made to acquire a parabolic figure, 

 (see Intellectual Observer, iii, 213,) and bore a power of 1000. The 

 winter was spent in perfecting the art of silvering and studying photographic 



