AN ACCOUNT 



OP 



THE CONSTRUCTION AND USE OF A SILVERED GLASS TELESCOPE. 



The construction of a reflecting telescope capable of siiovving every celestial 

 object now known, is not a very difficult task. It demands principally perse- 

 verance and careful observation of minutia?. The cost of materials is but triflin<r 

 compared Avitli the result obtained, and I can see no reason why sihered glass 

 instruments should not come into general use among amateurs. The future hopes 

 of Astronomy lie in tlie nudtitude of observers, and in the concentration of tlu^ 

 action of many minds. If what is written here sliould aid in the advance of tliat 

 noble study, I shall feel amply repaid for my labor. 



A short historical sketcli of tliis telescope may not be uninteresting. In the sum- 

 mer of LS57, I visited Lord Rossc's great reflector, at Parsonstown, and, in addition 

 to an inspection of the mucliinery for grinding and polishing, had an opportunity 

 of seeing several celestial objects through it. On returning home, in 1858, I 

 determined to construct a similar, though smaller instrument ; which, however, 

 should be larger tlian any in America, and be especial!} adapted for photography. 

 Accordingly, in September of that year, a 15 inch speculum was cast, and a 

 machine to work it made. In 1860, the observatory was built, by the village 

 carpenter, from my own designs, at my father's country seat, and the telescope 

 with its metal speculum mounted. Tliis latter was, however, soon after abandoned, 

 and silvered glass adopted. During 1861, the difficulties of grinding and polishing 

 that are detailed in this ac'count were met with, and the ri-iuedies for many of them 

 ascertained. The experiments were conducted by the aid of three 15| inch disks 

 of glass, together with a variety of smaller pieces. Three mirrors of the same 

 focal length and aperture are almost essential, for it not infrequently happens tliat 

 two in succession will he so similar, that a third is reqidred for attempting an 

 advance beyond them. One of these was made to acquire a parabolic figure, and 

 bore a power of 1,000. The winter was devoted to perfecting the art of sihering, 

 and to the study of special photographic processes. A large portion of 1862 

 was spent with a regiment in a campaign' in \'irginia, and but few photographs 

 were produci'd till autunm, when sand clocks and clepsydras of several kinds Jiaving 

 been made, the driving mechanism attained great excellence. During the winter, 

 the art of localcorrections was acquired, and two 15^ incli mirrors, as well as two 

 of 9 inclies for the photographic enlarging apparatus, were completed. The greater 

 part of 1863 has been occupied by lunar and planetary photography, and the 

 enlargement of the small negatives obtained at the focus of th(,> great reflector. 

 Lunar negatives have been produced which have been magnified to 3 feet in 



1 May, 1864. ( 1 ) 



