157 



this telescope, than of the large one; because the former 

 must be a greater segment of the sphere, than the latter. 

 For which reason, instead of making the one of an ellip- 

 tic, and the other of a parabolic form, I imagine it would 

 (with the exceptions before-mentioned) be more proper to 

 rest content with a spherical form in the little mirror, (by 

 which means, several of these latter, being fastened, with 

 cement, beside each other, on the same handle, might be 

 accurately and easily ground and polished together, on one 

 tool and polisher, made sufficiently large); and to employ 

 the great efforts on the large mirror, in rendering it of an 

 hyperbolic form; which is not at all more difficult than 

 it is to make it parabolic: for, on account of the small 

 extent of surface of the little min'or, it is very difficult 

 to govern and regulate its motion and pressure, so as 

 to communicate to it any certain figure, if polished 

 by itself singly ; as it must be, when it is to be of any 

 other than a spherical form. Yet, even this may, by an 

 intelligent and dexterous artist, be accomplished, to a 

 considerable degree of perfection, in the manner above- 

 mentioned, as I have repeatedly experienced; though the 

 process is much more easy and certain, in figuring the 

 large miri'or (under that limitation of its size before inti- 

 mated): for the greater the surface to be polished is, the 

 less Avill any inequality of pressure, in the operation, alter 

 the form of the mirror, or the polisher; such inequality, 

 being a part only of the motive force employed; and the 

 ■more extensive the surface is, the less proportion does the 

 ,^;ivoi. X. X motive 



