17G Silvered Mirror Telescopes. 



SILVERED MIRROR TELESCOPES— THEIR MERITS 

 AND DISADVANTAGES. 



The introduction of silvered mirror telescopes in this country 

 upon M. Foucault's plan, seems to have originated with 

 amateurs. A London optician, indeed, exhibited a few years 

 ago in his window a French pattern of small size in a square 

 mahogany case, and at a price considerably beyond its merits, 

 but it remained for Mr. Browning to make them, in an im- 

 proved form, a regular article of trade. Mr. Webb, in our 

 pages, was one of the first astronomers who directed the 

 attention of English observers to the advantages arising from 

 their construction, and to him belongs the merit of having 

 introduced Mr. Witt's mirrors to general notice and admi- 

 ration. Our readers will also recollect interesting commu- 

 nications from Mr. Bird, who constructed a large and fine- 

 silvered mirror telescope for his own use. A sufficient time 

 has now elapsed since Mr. Bird, Mr. Cooper Key, and Mr. 

 With turned their attention to these instruments, and since 

 telescopes with mirrors made by the latter, and admirably 

 mounted by Mr. Browning, have been in use, for some definite 

 replies to be given to numerous inquiries concerning the merits 

 and disadvantages of this construction. 



Two facts are now established, first, that the mirrors pro- 

 duced by Mr. With leave nothing to be desired in point of 

 accuracy of form. Weather permitting, and their mounting 

 being good, they are on the whole more, than equal to achro- 

 matics of the same aperture in point of dividing power and 

 definition, and in point of light they approximate to achro- 

 matics much more closely than the older telescopes with mirrors 

 of speculum metal, which are more easily affected, in an 

 unequal manner, by changes of temperature. A 6^-inch 

 silvered mirror in fair condition has more light than a 5-^ 

 achromatic, and readily divides stars which few achromatics 

 of its own aperture will touch. Thus, Mr. Slack's 6^ tele- 

 scope on good nights has several times distinctly split 

 7 2 Andromedae, with about 350, and this star is easy with 

 Mr. Webb's 9J. In the matter of cost the new telescopes 

 afford a happy contrast to the enormous price of fine achro- 

 matics — the larger sizes can be had, with fine equatorial 

 mountings, for a fraction of the cost of an achromatic object- 

 glass capable of doing the same work. 



The advantages of the silvered mirror instruments of the 

 Browning and With construction are cheapness, short focal 

 length, with corresponding facilities in use, absence of the 

 chromatic errors, of refractors, and absence of spherical 



