143 



allowed to state tlie difficulties, that, to my apprehension, 

 occurred in the enterprize, and to obviate objections; as, 

 from hence, there may be suggested some hiilts, to facilitate 

 or abridge future labour to others, or to prevent hopeless 

 trials. 



I must observe, then, that different effects miist neces- 

 sarily follow, from using, in the process of polishing, pitch 

 of a softer or harder consistence. If the pitch be of a 



T 2 temper 



day, been followed, observes, (to use his own words,) that " optic instruments 

 " might be brought to any degree of perfection imaginable, provided a ve- 

 " fleeting substance could be found, which would polish as finely as glass, and 

 " reflect as much light as glass transmits, and the art of communicating to it 

 " a parabolic figure be also attained. But there seemed [said he) very great 

 " difficulties, and / had almost thoiiglit them inmperahle, when I farther con- 

 " sidered, that every irregularity, in a reflecting superficies, makes the rays 

 " stray five or six times more out of their true course, than the like irregu- 

 " larilies in a refracting pne; so that a much greater curiosity would be here 

 " requisite, than in figuring glasses for refraction &c. 



" But, having afterwards thought on a tender way of polishing, proper for 

 " metal, whereby, as I imagined, the fgure would also be corrected to the last, 

 " (i. e. the utmost,) I began to try what might be effected in this kind ; and, 



•• by degrees, perfected an instrument &c and afterwards another 



" one." 



The tender way of polishing, which Sir Isaac Newton here mentions, was, 

 (as he afterwards described it in his Optics,) to cover the polisher with pitch: 

 and he declares, that he imagined \.\\e. figure, as well as the polish, would, by 

 means of this, be perfected. I cannot help thinking, that this extraordinary 

 man, who was born to anticipate others in invention, as well as discovery, 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 Prin- 

 cipia,) that a spherical mirror will reflect the oblique pencils, issuing from 

 the extremities of the field of view, as truly as a parabolic one, and seems tto 

 despair of effecting a more correct figure. 



