REFLECTING TELESGOPff. 45 



"eiftinent opticians, as well as artists, that have laboured In the 

 improvement of this instrument;) I have not heard, that any 

 method has been proposed, of communicating, to the little 

 rairror of the Gregorian telescope, any other than a spherical 

 form, which yet may in this manner be done. And it must,, 

 in this telescope, be a thing most desirable to accomplish ; 

 especially when its size and aperture is so great, that it 

 would be difficult to impress, on the extensive surface of its 

 great mirror, (merely by the small alteration of figure, which 

 could be produced, in the delicate operation of polishing,) 

 the degree of change, from its prior state of spherical curva- 

 ture, which would be requisite ; since the defect of form, m 

 this mirror, may, in these cases, (as will be shewn,) be easily 

 compensated, in the figuration of the little mirror. For the 

 greater size of this latter, in such instances, will render it 

 capable of more steady handling and motion, and more equal 

 pressure ; and so more manageable, and susceptible of a cor- 

 irect figure, in proportion as the encreased magnitude of the 

 great mirror renders it unmanageable : which is, plainly, a 

 great advantage, in the fabrication of this telescope; whose 

 mirrors will thus, in the cases where it is most especially ne- 

 cessary and desirable, adrnit mutual correction and compensa- 

 tion fpr each other's defects. 



The principles, or physical causes, operative in this process, Difficulties of 

 as above stated, seem to be incontrovertibly evident; and, as ^^^ piocesss. 

 .:! am not aware of any paralogism admitted in the reasoning 

 upon them, I must suppose, that a mode of operation, con- 

 formable to these principles, is the thing chiefly requisite tb 

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

 the process; and, as the almost insuperable difficulties attend- 

 ing it are felt, even by those whose 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, may be allowed to state the 



* Sir Isaac Newton, who had himself laboured in this under- 

 taking, of polishing the concave mirror of his own telescope, and, 

 wiih such talents for the work, and such success, as to discover 

 that method of doing it, which has, to this day, been followed, 

 observes, (to use his own words) that " optict instruments 

 '* might be brought to any degree of perfection iniaginable, pro- » 



*' vided a reflecting substance could be found, which would' po- 



