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French Institute. 

 LXI. Report on the ponderous Flint Glass intended for the 

 Manufacture of Achromatic Glasses. Presented to the In- 

 stitute by M. DouEourgerais, Optician to His Majesty 

 the Emperor and King.—Laid before the Class of Mathe- 

 matical and Physical Sciences, by Messrs, De Prony, 

 GuyTon, and Rochon, onTucsday April 10, 180y. 



We know that the invention of achromatic glasses is one 

 of the grandest discoveries of the last century. We are in- 

 debted for it to a celebrated geometrician, who has enriched 

 the mathematical sciences with the most astonishing con- 

 ceptions. In 1747 Euler entertained the sublime idea of 

 correcting, by the employment of several diaphanous sub- 

 stances, the aberration resulting from the decomposition of 

 light in spherical glasses. 'J his was the more valuable, be- 

 cause philosophers had been led to believe, according to the 

 experiments of Newton, that there was no refraction when 

 there was no dispersion, — thus" banishing all hope of destroy- 

 ing the colours in glasses. 



Euler informs us, in the Memoirs of th* Academy of Peters- 

 burgh, that some experiments made upon meniscus glasses, 

 the concavities of which he filled with various liquors, proved 

 that the different refrangibility of the rays of light could be 

 diminished, and even reduced to nothing, (these are the ex- 

 pressions of this great man, whose modesty was equal to 

 his talents,) by employing two or more transparent sub- 

 stances. He adds, what is very remarkable, that the won- 

 derful structure of the eyes, which represent the images of 

 objects from their posterior extremity, suggested to him 

 that it would be possible to diminish, and even to annihilate, 

 all the defects to which the different refraction of the ravs 

 of light at that time seemed to be necessarily subjected. 

 " Here again, " says Euler, " we recognise the power of the 

 Deity, as well as his infinite wisdom." He informs us at 

 the same time, that his opinion was attacked by John Dol- 

 lond, an eminent optician of London : but upon some ob- 

 servations of M. Klmgeiistierne, he ascertained, after amul- 

 Vol. 33. No. 133. May 1809. Y tiplicity 



