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XXXII. Queries respecting Mr. Hall's original Discover!/ of 



Achromatic Telescopes. By A Correspondent. 



To the Editors of the Philosophical Magazine and Annals. 



Gentlemen, 



I HAVE for several years observed in the chronological 

 table of the original construction of astronomical instru- 

 ments, published in the valuable French Almanac, entitled 

 « Anmiaire presente au Roi par le Bureau des Longitudes" 

 that the first achromatic telescope is stated to have been con- 

 structed by Mr. Hall in 1750; and the publication of the dis- 

 covery of achromatic telescopes by Mr. Dollond is dated 

 eight years subsequendy, or in 1 75S. — As few of our English 

 writers on optics have ever mentioned the name of Hall, his 

 merit, as the original inventor of the achromatic telescope, is 

 almost unknown in the country where the discovery was first 

 made. The discovery is indeed faintly alluded to in a note 

 by Dr. Young in his Lectures; and a reference is made to the 

 Philosophical Magazine for November 1798, where I find a 

 more ample account of Mr. Hall's telescopes ; but the infor- 

 mation is still confined to a note, and very brief: it becomes, 

 however, extremely valuable from the testimony of the late 

 Mr. Ramsden, that it contains a true statement of the facts 

 relating to the discovery. 



The attention of astronomers is at present directed to the 

 improvements lately made by opticians on the continent in 

 achromatic object-glasses, which are now constructed in a 

 perfect manner, with apertures far exceeding any that have 

 been made from English glass, and which will probably su- 

 persede entirely the use of large reflecting telescopes. The 

 mirrors of the latter, beside their liability to tarnish, have their 

 figure injured by their own weight when they exceed two feet 

 in diameter. 



It will doubdess be acceptable to many of your readers to 

 republish the kw but decisive facts at ])resent known respect- 

 ing Mr. Hall's iujportant discovery, to which I shall subjoin 

 some ([ucries ; the answers to which would gratify the astro- 

 nomers of Uiis country, and tend to render justice to a gentle- 

 man whose merits have been unaccountably neglected. " The 

 inventor was Chester More Hall, Esq. of More Hall, in Essex." 

 It appears from his papers, tliat he commenced his labours in 

 tlie year 1729; and afler many experiments, he had the good 

 fortune to find two sorts of glass which had the recjuisite pro- 

 l)ertics for dispersing the rays of light in contrary directions 

 wlien formed into lenses, in order to show objects colour- 

 less. 



N. S. Vol. G. No. 3.'}. Sept. 1 R29. 2 H " About 



