142' 



tion of polishing,) the degree of change, from its prior state 

 of splierical curvature, which would be requisite; since the 

 defect of form, in this mirror, may, in these cases, (as Avill 

 be shewn,) be easily compensated, in the figuration of the 

 little mirror. For the greater size of this latter, in such in- 

 stances, will render it capable of more steady handling and 

 motion, and more equal pressure; and so more manageable, 

 and susceptible of a correct figure, in proportion as the en- 

 creased magnitude of the great mirror renders it unmanage- 

 able : which is, plainly, a great advantage, in the fabrication 

 of this telescope; whose mirrors will thus, in the cases where 

 it is most especially necessary and desirable, admit mutual 

 correction and compensation for each other's defects. 



The principles, or physical causes, operative in this pro- 

 cess, as above stated, seem to be incontrovertibly evident; 

 and, as I am not aware of any paralogism admitted in the 

 reasoning upon them, I must suppose, that a mode of ope- 

 ration, conformable to these principles, is the thing chietly 

 requisite to ensure success. In this view, I have attempted 

 to conduct the process; and, as the almost insuperable dif- 

 ficulties attending it are felt, even by those Avliose inventive 

 powers and resources ought to afford the highest hopes of 

 accomplishing the object, and yet disappoint them in their 

 attempts at high perfection ;* so I, among others, ma}' be 



allowed 

 1 1. '■ • . ' 



* Sir Isaac Newton, who had liimself laboured in this undertaliing, of polishing 

 file concave mirror of his own telescope, and with such talents for tiie work, 

 -:in<l such success, as to discover that method of doing it, which has, to this 



