45 REFLECTING TELESCOPE. 



difficurties, that, to my apprehension, occurred In the errter"' 

 prize, and to obviate objections ; as, from hence, there may be 

 suggested some hints, -to facihtate or abridge future labour to 

 others, or to prevent hopeless trials.- 

 Argumentsand I niust observe, then, that different effects must necessa- 

 inferences res- j-ily follow, from using, in the process of polishing, pitch of a 

 process of po- softer or harder consistence. If the pitch be of a temper quite 

 lishing mirrors, hard and unyielding, no j)art of the surface of the mirror can 

 be made to suffer a higher degree of friction than the other 

 parts of it, unless these latter parts be elevated and detached 

 from the face of the polisher, and disengaged from contact with 

 it ; because. In this case, both mirror and polisher are sup- 

 posed to preserve their general shape regular and unaltered j 

 and therefore, the contact, and, consequently, the friction, 

 must be either complete and equal, on the whole surface, or 

 none at all. For, if we suppose, that, by the wearing down 



" lish as. finely as glass, and reflect as much light as glass trans- 

 •' mits, and the art of communicating to it a parabolic figure be 

 " also attained. But there seemed {said he) very great difficulties, 

 " and / had almost thought them insuperable, when I farther 

 ** considered, that every irregularity, in a reflecting superficies, 

 •* makes the rays stray five or six times more out of their true 

 " course, than the like irregularities in a refracting one ; so that 

 " a much greater curiosity would be here requisite, thaninfigur- 

 " ing glasses for refraction, . . . , &c. 



*' But having afterwards thought on a tender way of polishing, 

 " proper for metal, whereby, as I imagined, the figure would also 

 *' be corrected to the last, (i.e. to the utmost) I began to try what 

 *' might be effected in this kind ; and, by degrees, perfected an 

 " instrument .... &c and afterwards an otherone." . 



The tender way of polishing, which Sir Isaac Newton her« 

 mentions, was (a<i he afterwards described in his Optics,) to 

 cover the polisher with pitch ; and he declares, that he imagined 

 ihejftgure, a's -well as the polish, would by means of this, be per- 

 fected. I cannot help thinking, that this extraordinary man, • 

 who was born to anticipate others in invention, as well as disco- 

 very, had the same ideas as are here detailed, though he did not 

 explain, nor, perhaps, succeed in, the application of them in 

 practice: for he states, (in his Principia) that a spherical mirror ^ 

 will reflect the oblique pencils, issuing from the extremities of the 

 £eldof view, as tiuly as a parabolic one, and seems to despair of 

 effecting a more correct figure. 



