178 



I have, in tliis method, with ceilainty of success, as ve- 

 rified by examination of the progressive change of curva- 

 ture in the min'Oi's, fi-om a greater degree -to a less, and 

 vice versfi, eftected the desired configuration of them: 

 ■vvliich serves to confirm me in the behef, that the circum- 

 stances, above proposed, are those -which are teally ope- 

 rative, in communicating the diversity of figure to teles- 

 copic mirrors ; and that neither the direction of the strokes 

 in .polishing, the size or form of the polisher, consistence 

 of the pitch made use of, or other accidents, are of any 

 farther moment in the process, than as they serve to mo- 

 dify the resistance of the pitch, in the several parts of the 

 surface of the polisher. Whether, by attention to the 

 principles here laid down, it Avould be possible to pro- 

 duce an hyperbolic form, in the convex mirror of the 

 Cassegrain telescope, I ^have been j^revented from endea- 

 vouring to ascertain by experiment, from those casualties, 

 aft'ecting my situation in life, which I have already in- 

 timated. But it should seem, that it would, to a cer- 

 tain degree, be pi-acticable, from the means I have sug- 

 gested, of producing a progressive, specific alteration in 

 the figure of the polisher; if I have judged rightly of 

 the existence and caivse of ^that alteration. 



And if it should be found possible to give, to a convex 

 speculum, an hyperbolic curvature, the same could be done 

 to the convex object glasses of dioptrical telescopes: which 

 is a property still wanting to them.; a want which makes 

 them inferior to reflectors, even when they have been ren- 

 dered 



