MEMOIR ON ACHROMATIC GLASSES. ||J 



knew they were for the lame perfon, flievved them to Dollond, 

 ibr whom likewife he worked, and who thus learned what he 

 afterwards publifhed as his own dilcovery. 



Be it as it may, I fliall with difficulty be perfuaded, that But Euler the 

 Euler was not the firft who thought of correcting the aDerra " J ^ p C r °indpIe 

 tions of refrangibility, as Newton invented the means of cor- 

 recting the aberrations of fphericity, by compofing an object 

 glafs of glafs and water. Was it not the illuftrious Euler, to which he was 

 who, reflecting on the itructure of the eye, fufpected this or- l !! d *™ m re " 

 gan was compofed of different mediums for the purpofe of de- ft ru & ure f the 

 Itroying the confulion produced by the decomposition of light eye. 

 when it traverfes a fingle medium ? And did he not publifh 

 in the Memoirs of Berlin and of Peterfburg, that fublime idea, 

 which accorded with the fyftem of perfection he afcribed to all 

 the works of the Supreme ? It is true, the inutility of this cor- Though the 

 rection of the aberrations for the fphere of diftinft virion, from co _ rr ^ lon he 

 the fliortnefs of the form of the eye, may be objected to Euler*s ft ruc ~t ure f the 

 hypothecs : but this makes nothing againft his claim to the dif- e y e not necef- 

 covery, to which he was led by it. Neither had this happy ^L^ 

 idea that fuccefs at the time, which might have been hoped His idea neglect- 

 from it j partly becaufe the theory of this great geometrician ed > becaufe his 

 was founded on laws of refraction purely hypothetical; partly p thetica? S and" 

 becaufe it was repugnant to the affertion of Newton, that, contradictory to 

 when light traverfes feveral mediums of different natures, fo Newton?" ° f 

 that the emergent rays are parallel to the incident, the light is Klingenftiem 

 not decompofed. It is well known, that the learned Klingen- exprefTed his 

 ftiern, in 1755, exprefTed his doubts of the laws of refraction ^laws^" 

 advanced by Newton : and that thefe doubts were confirmed refraction j 

 in 1759, by Dollond's experiments on crown and flint glafs, and Dollond 

 which firft exhibited to the learned colourlefs prifms. doubts. 



It was in the beginning of 1774, that I read to the academy Rochon pro- 

 a memoir, in which I propofed to improve achromatic object P ofes t0 im - 

 glafles by the interpofition of a fluid between the two lenfes of compounded 

 crown and flint glafs which compofe them. Borda, le Gentil, and ? b J e,a g^fs, by 

 Caffini, were appointed a committee to repeat my experiments. fl ul j between 

 They obferve in their report, that, " in an object glafs of three the pieces, 

 lenfes, if there be a difference of a thoufandth part of a line Re port of Bordt 

 between the curvature of the centre and that of the edges of t ^j s fubjec"t°. n 

 each furface, a fenfible imperfection in the image of the object 

 will be the confequence. But the mere heat of the workman's 

 land, in giving the glafs the finifliing polifh, is capable of di- 



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