(34 . Mechanical Philosophy. [Chap. I. 



nal advantages of construction seem to have been 

 devised from the time of Huygens, till the middle 

 of the eighteenth century. It was then that Mr. 

 Dollond, a celebrated artist of Great Britain, dis- 

 covered a method of correcting the inconvenience 

 and errors arising from the different refrangibility 

 of the solar rays ; a difficulty, which, since the time 

 of Newton, had been generally considered as in- 

 surmountable. He ascertained that lenses of crown 

 tmd o^ flint glass might be so prepared and adjust- 

 ed as to correct the refractive, and of course the 

 chromatic, powers of each other. On this disco- 

 very he founded the construction of his celebrated 

 Achromatic Telescope, which, doubtless, deserves 

 to be ranked among the most valuable acquisitions 

 of the age*. Mr. Dollond pursued this improve- 

 ment by increasing the number of glasses, with so 

 much success, as to make the refracting instru- 

 ments of his time superior to the reflecting of equal 

 length. The principle which he discovered was 

 explored still further, and the Telescope which he 

 contrived carried to a higher degree of perfection 

 soon afterwards, by Mr. Zeiher, of Petersburgh, 

 who ascertained, by experiments, that increasing 

 the quantity of lead in the formation of lenses, 

 augmented the power so much desired in this in- 

 strument. It ought, perhaps, to be mentioned, 

 that the hints and publications of the celebrated 

 Euler on this subject, though found erroneous, 

 probably contributed something to Mr. Dollond's 

 discovery; and that the distinguished mathemati- 

 cians, Clairaut and d'Alembert, about the same 



* Sfce Additional Notes — (0), 



