338 Report on the ponderous Flint Glass intended 



tiplicity of experiments, that the great inequality of the 

 dispersive powers which takes place in two kinds of glass, 

 vulgarly known by the names of flint and cronn glass, 

 was sufficient for realizing the idea of Euler, and thereby 

 good achromatic glasses were obtained. Dollond's success 

 procured him a patent in 1759, wich was called in question, 

 however, by Valtines in Westminster Mall. Valtines proved 

 that the ingenious Chester Morehall had constructed glasses 

 long before Dollond, perfectly achromatic, and of an im- 

 mense amplifying power. So early as the year 1754, M. 

 Ayscough, an optician of Lancaster, possessed one of these 

 instruments, as did also Dr. Smith. These facts, although 

 but little known, deserve to be published, and were authen- 

 ticated by lord Mansfield, who maintained Dollond in his 

 privileges, merely because the benefit of a patent does not 

 belong to him who has the first scientific idea of an inven- 

 tion, but to him who enables the public to enjoy the ad- 

 vantages of the discovery. 



So far Mr. Dollond deserved a recompense; and the cele- 

 brated achromatic instrument with a triple object glass which 

 lue presented to the Royal Society excited a great sensation 

 in the scientific world. 



The Academy of Sciences, on being informed that instru- 

 ments were made at London upon the principles of Euler, 

 and which magnified one hundred times the diameter of the 

 objects with the degree of clearness and distinctness requisite 

 in delicate observations, proved by ingenious inquiries, that 

 it attached the highest possible value to the new discovery 

 in question. Two eminent geometricians, Messrs. Clairaut 

 and D'Alembert, have left nothing to desire upon the in- 

 tricate theory of the construction of these instruments. 

 They fixed the spherical curvatures of glasses of unequal 

 dispersive forces, which reduce to the minimum the aberra- 

 tions of refrangibility and sphericity. 



M. Clairaut afterwards ascertained by experiments, Chat 

 the lapidaries of Paris, who endeavoured to imitate the dia- 

 mond in their glassy compositions, made use of a kind of 

 glass vulgarly known by the name of strass ; the dispersive 

 power of which is greater than that of flint glass. But this 



glass, 



