£76 Obfervations on Platina, &c. 
Aer’s object-glafs, with water and glafs like that which News 
ton had imagined, to render the aberration of fphericity. as 
little as poffible; but this objeét-glafs did not fucceed, on 
account of the proportion fince known between the refrac 
tion and difperfion of common glafs and water. This pro- 
portion is by the diafporameter as 155 to 133 in regard to 
_refraction, and as 100 to 67 in regard to difperfion. But 
he ftrong curvatures which muft be given to deftroy the 
-aberration of refrangibility would occafion a very ftrong 
aberration of fphericity. Thus, what might be gained on 
‘one hand would be infallibly lof on the other. This was 
fufficient to make Euler’s project. mifcarry, and to confirm 
the aflertion of Newton, which tended to deftroy all hope of 
being able to.obtain achromatifm in glafs. Since. that 
_epoch the firft knowledge of the ftrong difperfion of glafs in 
the compofition of which: there is much lead, and which in 
England is known under the name of fliné gla/s, has been 
afcribed to John, Dollond. Tihs appellation however is not 
_that beft fuited to this kind of glafs. But what is of moft 
“importance to be known here is, that the more lead, or 
rather minium, is employed in making glafs, the more will 
its difperfive power be augmented. It was in the year 1759 
that Dollond prefented achromatic glaffes, compofed of flint 
and’ crown glafs. He fays in his paper printed in the Philo- 
fophical Tranfactions, that he found means to deftroy with 
great eafe the aberrations of refrangibility ; but he confefles 
that he was ftopped by an object more dificult to befur- 
mounted, that of annihilating at the fame time the aberration 
_of fphericity. It was thought, and ‘is ftill believed in France, 
that John Dollond was the inventor of achromatic telefcopes ; 
but we know that, fome years after, Dollond’s patent was at- 
tacked, on the ground that he was not really thei inyentor*, 
AIT. 
* The author here goes on to give what he confiders a correct account 
of the origin of the inyention ; but as he is incorre& in almoft every otr-— 
‘ cumftance, 
——_ 
