SS REFLECTING TELESCOPE. 



The radial and employed, nor the shape of the area of the polisher, which, iuv 

 stroke'com- reality, produces a conoidal form in the mirror; but a gradual 

 pai«d. alteration in the curvature of the face oi the polisher, by yield- 



ing of the pitch, under the pressure. And, therefore, when 

 any part of the area of the polisher, whether it be round or 

 oval, is more extended than that of the mirror ; the pitch, 

 moving laterally, will become elevated, and its curvature les- 

 sened, in that part. So that, in a polisher of oval shape, whose 

 conjugate diameter is equal to that of the mirror, the pitch 

 will ascend and accumulate, in the patt, which lies without the 

 circular area of the mirror, inscribed in the ellipse. The ex^ 

 tremities of the mirror will, therefore, be worn down, when 

 each part of them is made in rotation, by straight strokes across 

 the polisher, in the transverse diameter of the ellipse, to tra-» 

 verse that part of it, which circumscribes the circle ; and, by 

 such strokes made twice, directly in that diameter, and oftener 

 obliquely, in each rotation of the mirror, as the operator moves 

 round the polisher, during the process, the regular shape of the 

 polisher is preserved. Bat it is easy to conceive, that the 

 same effects would follow, though the polishing were.vCon- 

 ducted, not by straight strokes across, but by round strokes, in 

 a spiral direction, as above mentioned. Apd I am doubtful, 

 to which of these motions the preference should be given; or 

 whether they ought not to be interchangeably used, to pro^ 

 duce the most elaborate form in the mirror ; as also, whether 

 this method, of Mr. Edward^, is better than the former, by Mr. 

 Mud'^e, above described. For I have been deprived of lei- 

 sure and opportunity (by the war, and public troubles, during 

 the French invasion and the rebellion; in which, most of my 

 instruments, for such purposes, were lost, in the plunder and 



Ponbt which 

 sway be pre- 

 krablc. 



tool, which he calls a bruiser ; by which he must have preserved 

 or recovered, the figure of the polisher, and, consequently, of the 

 mirror, that otherwise must have become vitiated, by the unequal 

 resistance of the pitch ; and Mr. Edwards made furrows in the 

 coating of pitch, or his polishers. It is to these circumstances, 

 and not to the direction of the motion employed, or the elliptic 

 area of the polisher, that, I can think, was owing to the success, 

 attendant on their methods: the bruiser being necessary, to supply 

 the defect of furrows in the pitch ; and the oval form not essential, 

 when there were such, duly disposed, and also the polisher ot 

 proper size, &c. as here directed. 



