5 4 Mech an ical Ph ilosophy . 



ing than any hitherto mentioned. Much was 

 done, during the period in question, in the improver 

 ment of Telescopes. The Befr acting Telescope was 

 first in use. In this instrument no signal advant- 

 ages ofx construction seem to have been devised 

 from the time of Huygens, till the middle of the 

 eighteenth century. It was then that Mr. Dol- 

 XAND, a celelvated artist of Great-Britain, disco- 

 vered a method of correcting the inconvenience 

 and errors arising from the different refrangibilitv 

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

 of Newton, had been generally considered as in- 

 surmountable. He ascertained that lenses of 

 crozon and of flint glass might be so prepared and 

 adjusted as to correct the refractive, and of course 

 the chromatic pov/ers of each other. On this dis- 

 covery he founded the construction of his cele- 

 brated Achromatic Telescope, which, doubtless, 

 deserves to be ranked among the most valuable 

 acquisitions of the age. Mr. Dolland pursued this 

 improvement by increasing the number of glasses, 

 wth so much success, as to make the refracting 

 instruments of his time superior to the reflecting 

 of equal length. The principle which he disco- 

 vered was explored still flirther, and the Tele- 

 scope which he contrived carried to a higher de- 

 gree of perfection soon afterwards, by Mr. Zei- 

 HER, of Petersburgh, who ascertained, by experi- 

 ments, that increasing the quantity of kad in the 

 formation of lenses, augmented the power so much 

 desired in this instrument. And, still more re- 

 cently, additional discoveries have been made re» 

 specting it, and additional means of correcting its 

 inaccuracies devised, by Mr. Robert Blair, Pro- 

 fessor of Astronomy in the University of Edin- 

 burgh.' It ought, perhaps, to be mentioned, that 



t See Transactions of the Royal Society of Edinburgh, vol. ili. 



