for the Manufacture of Achromatic Glasses* 33£) 



glass, to which an artist of the name of Strass had given, by 

 means of the oxide of lead, a gravity equal to that of the 

 diamond, is generally so soft {gelalineux) , that it is very diffi- 

 cult to succeed in employing it usefully in the manufacture of 

 achromatic object glasses, which require glasses not only 

 perfectly homogeneous, but also blown glasses, according 

 to the remarks of the most eminent opticians, who have 

 discovered, in the practiee of their art, the advantages of 

 these blown glasses over those which were run into crucibles. 



M. Loysel, in his Essay upon Glass Making, gives us 

 the composition of a glass imitating the dispersion of the 

 diamond : it is, he informs us, with 100 parts of white sand, 

 washed in muriatic acid, combined and fused with 150 parts 

 of red oxide of lead, to which must be added 30 parts of 

 aerated and calcined potash, and ten parts of calcined borax, 

 that the lapidaries produce, in small furnaces, the crystal 

 which imitates the diamond, and which has the same 

 weight ; its specific* gravity being as to water 35 to 10. They 

 sometimes add one part of oxide of arsenic ; but this com- 

 position, which they allow to cool in crucibles, produces 

 but very small masses, which can only be used in making 

 trinkets. 



If, in the origin of the invention of achromatic glasses, 

 M. Clairaut made use of this glass in the construction of 

 some achromatic object glasses, it was because l7e was de- 

 sirous to make an useful application of his formula upon 

 glasses the dispersive power of which was much greater 

 than that of flint glass : but M. de l'Etang, whom he charged 

 with this work, remarked to him that blown glass, such as 

 flint and crown glass, was requisite for good object glasses. 

 On this account the Academy of Sciences, at the suggestion 

 of the government, who were unwilling that France should 

 be tributary to England in the article of glass, proposed in 

 1766, as the subject of a prize dissertation, the best process 

 for imitating in France a ponderous glass exempt from all 

 defects, and having all the properties of flint glass. 



This prize was granted in 1773 to M. Lebaude, the ma- 

 nager of a glass-work, and his Memoir was printed among 

 the Memoires des Savaus Etrangeres fur 1774. 



Y 2 M, LebanJe, 



