fur the 'Manufacture of Achromatic Classes. 341 



capital generously sacrificed for the attainment of an object, 

 the importance and utility of which he knew how to ap- 

 preciate. 



We shall now give an account to the Institute of the na- 

 ture and quality of the ponderous glass which has been sub- 

 mitted to our examination. 



We ought to premise, that the most eminent opticians are 

 fully satisfied of the qualities of the glass in question, and 

 that a great number of achromatic telescopes have been made 

 with it. We now call the attention of the Institute to ^ 

 letter to M. Doufourgerais from M. de Freminville, chief 

 engineer of roads and bridges, who is specially charged with 

 furnishing the telegraphs and the navy with the glasses re- 

 quired for the observation of signals. 



" Pieces of glass from your magazine, taken at random, 

 and subjected to the necessary operations for employing them 

 as optical glasses, produced object glasses comparable to the 

 best of Dollond's of the same dimensions. You have there- 

 fore attained, and I am proud to bear witness to it, the 

 highest degree of perfection ever possessed by English glass, 

 whether I consider it in a commercial or scientific point of 

 view, since in your article beauty and utility are united to 

 cheapness." 



This impartial testimony from a person well acquainted 

 with optical instruments, we are happy in being able to 

 corroborate from our own experience. The glass made by 

 M. Doufourgerais is heavier than flint glass: one of us 

 measured the gravity of the former in the hydrostatic ba- 

 lance, and found it to be 3,588 with respect to distilled 

 water, while the heaviest flint glass is only 3,329. 



A prism of the glass made by M. Doufourgerais, having 

 an angle of two degrees, ceases to colour objects the instant 

 we place it against a prism of common glass, (such as the 

 blown glass made at Cherbourg, which differs very little 

 from crown glass,) when its angle is 18 degrees : from the 

 experiments therefore of one of your committee, it appears 

 that the dispersion which takes place in the glass made by 

 M. Doufourgerais, is, to that observed in the most ponderous 



Y 3 Aim 



