HO MEMOIR ON ACHROMATIC GLASSFS. 



X. 



Memoir on Achromatic Glajfes adapted to the Meafure of Angles f 

 and the Advantages that may he derived from double Refraction 

 for the precife Meafurement of fmall Angles: by Allxis 

 Roc hon, Member of the National Injlituie, and Director of 



. the Naval Obfervatory at Brcjl *. 



Jta?*tt£ ° f W'hEN we would determine thepofition of places, either 



mined by angles, on the earth or in the heavens, our conclufions are always 



founded on angles obtained by obfervation ; and of courfe the 



certainty of our conclufions muft depend on the precifion with 



Euler faid to which we can meafure thefe angles f . Euler was the firft who 



have firft at- attempted to correct the aberrations of refrangibility, by em- 

 tempted to cor- r # ° J J . 

 re£k the abeira- ploying fubftances of different refractive powers. Maupertius 

 tions of refran- p roC ured Euler's object glafs, confifting of glafs and water, to 

 gibility by a lens v . \ ° i j . r a 

 of glafs and De made at rans : but this we now know could not iucceed, 



waterj from the ratios the refractions and difperfions of common glafs 



and water bear to each other. 

 and afterward In 1755, Mr. Dollond laid before the public his achromatic 



Dollond by flint bje& glades, compofed of flint glafs and crown glafs. He 

 Stafs - anC " ky s * n n ' s P a P er m tne Philofophical Tran factions, that he 

 deftroyed the aberrations of refrangibility with tolerable eafe ; 

 but to deftroy that of fphericity at the fame time, was an ob- 

 an invention je<5fc much more difficult to furmount. For this Dollond ob- 

 wl, ' ch h ^" fald tained a patent, the exclufive privilege of which was attacked 

 from Hall. by Mr. Watkins of Charing Crofs, on the ground that the dif- 

 covery had been made long before by a gentleman of the name 

 of Hall, who however had not rendered it public. It is faid 

 that Mr. Hall gave orders for his lenfes of flint glafs to an op- 

 tician in one part of London, and for thofe of crown glafs to 

 another in a diftant part, that his fecret might not be dif- 

 covered ; and that both of them happened to employ the fame 

 workman to make them, who fufpecting fome myitery, as he 



* Abridged from the Journal de Phyfique, Fru&idor, year 9. — C. 



f The hiftory of the invention of achromatic glafTes is given very 

 erroneoufly in this and moft foreign memoirs. I purpofe foon to 

 give a commentary on this fubjeft,— -N. 



knew 



