VOL. LXVII.] PHILOSOPHICAL TRANSACTIONS. 1 Sq 



2 foot metal, which bore a charge of 200 times, only by opening the sharp part 

 of tlie edge of the hole, because I thought it bounded the tield: so essentially 

 necessary is an exquisite correctness of figure in the speculum of a perfect 

 reflector. 



This experiment not succeeding, instead of casting the metal without a hole, 

 1 made one quite through the middle of the polisher, a little less than that in 

 the speculum. This perfectly answered the purpose; no more inconvenience 

 arose from the gathering of the pitch, for it had now no greater tendency to 

 collect at the centre than the sides; and 1 finished several metals successively, 

 excellent both in point of figure and polish; one of those of 2 inches diameter 

 and 7.5 focal length, bore a charge of 6o times, and upwards. This telescope 

 underwent Mr. Short's examination, who was pleased to remark, only, that he 

 thought he had made one more distinct. 



I must observe that, in this method of working, the polishing goes on in an 

 agreeable, uniform, and smooth manner ; and that the small degree of yielding 

 in the pitch, which is actually not more than the wearing of the metal, pro- 

 duces that mutual accommodation of surfaces so necessary to a true figure. In 

 the beginning of the polish, and indeed for some time during the progress of it, 

 always remembering now and then to move the metal round its axis, I worked 

 round and round, not far from and always equally distant from the centre, 

 except that every time, previous to the shifting the metal on its axis, I used a 

 cross stroke or two; and when the polish was nearly completed, I mostly used 

 cross strokes, giving a round stroke or two likewise every time I turned the 

 metal on its axis. I observed in this method of working, that the metal always 

 polished fastest in the middle; insomuch, that half or •§- of it would be com- 

 pletely polished when its circumference was scarcely touched by the tool. 

 Observing this in some of the first metals, and not considering that this way of 

 polishing was in fact a species of grinding, and as perfect as that on the hones, 

 I went on reluctantly with the work, almost despairing of being able to produce 

 a good figure. However I always found myself agreeably deceived; for when 

 the polish was extended to the edge, or within the lOth of an inch of it, I 

 almost constantly found the figure good, and the performance of the metal very 

 distinct. But this same circumstance of apparent defect in the metals, was in 

 fact that to which their perfection was owing; for they all, contrary to expecta- 

 tion, turned out parabolic. However I did not for a great while know any 

 certain way of giving that degree of parabolic tendency which was just necessary, 

 and which will be described hereafter. It was a long time before I got rid of 

 my prejudice against this apparent imperfection in the process, or could recon- 

 cile myself to the irregular manner in which the polish proceeded; for I con- 

 sidered it as a certain source of error, and though I saw it eventually succeed, 



VOL. XIV. Z 



