Tur Author of the Paper “On Achromatic Eye-pieces,” &c., has to 
apologize for an error in his statement of the principle of Dr. Blair’s 
object-glass, (in Article 7 of that Paper, page 239). The construction 
finally adopted by Dr. Blair was, to inclose a mixture of muriatic acid 
with a muriatic salt between two lenses of the same sort of glass. By 
varying the proportions of the mixture it was found possible to form a 
ying prop P 
fluid in which — was exactly expressed by the formula c on 
