Sect. VII.] Optics. (iJ 



time, rendered themselves very conspicuous among 

 the philosophers of Europe, by their ingenious cal- 

 culations and suggestions, in aid of the achromatic 

 instrument. 



A new method of constructing ^^c//;v;wrt/zc Tde- 

 scopes was discovered, a few years ago, by professor 

 llobert Blair, of Edinburgh. This method consists 

 in the use of one or more Jliu'd mediums, of which 

 the dispersive powers, being opposed to each other, 

 correct the focal irregularities not only of the ex- 

 treme rays of the Newtonian Spectrum, but like- 

 wise of those near the middle; to which former 

 opticians had little, if at all, attended. From some 

 cause or other, however, this kind of achromatic 

 telescope has not been much used. 



But, from the necessary imperfection, and tlio 

 small limits to which the dioptric plan of magnify- 

 ing distant objects is confined, the improvement 

 of the Reflecting Telescope became, early in tlie 

 century, an object of particular attention. This 

 instrument, which had been invented, in the pre- 

 ceding centuryj by Mr. James Gregory, of Aber- 

 deen, and which had been executed, on a different 

 plan, by sir Isaac Newton, was greatly improved 

 by Mr. Hadlej', who, in 1719, presented a very 

 powerful Telescope of this kind to the Royal So- 

 ciety. In 1734 Mr. Short, an ingenious artist of 

 Edinburgh, devised still further improvements in 

 this instrument. These were chiefly eilected by a 

 new method which he discovered of grinding mir- 

 rors. But the secret art which enabled him to 

 do this with so much success, is said to have died 

 witli him. After, him, Mr. Mudge, of Plymouth, by 

 making specula of a composition uf dillcreut metals. 



