168 



If the mirror be of an elliptic, or oval, curvature, rays, 

 issuing from single points, in, or extremely near to one of 

 its foci, and falling on it, (such as the x'ays proceeding 

 from the single points in the image, formed by the para- 

 bolic larger speculum,) will be converged to so many single 

 distinct points, in the other focus of the elliptic mirror. 



Hence, 



sible magnitude: for the pencils of rays that issue, diverging from each poin* 

 of the printed letters, will be reflected, by the central part of the mirror, to 

 a focus nearest to it; and the rays of each pencil, that fall on the exterior 

 annuli of the mirror, will be reflected to points more remote from it. So 

 that if, in the Newtonian and Gregorian telescope, the great mirror were of 

 the correct figure of a parabola, and the little mirror, of the latter, were that 

 of an ellipsis: and, if either telescope were adjusted to distinct vision, when 

 the innermost zone only of the great speculum is exposed to the light ; yet, 

 the object would be indistinct, if seen by the rays reflected from the outer 

 zone, unless the little mirror were removed further from the great ,one. 

 Hence, a spherical mirror is better than a parabolic one, for viewing very- 

 near objects; and neither of them can be equally adapted for viewing these 

 and very remote objects. The distinctness of the telescope is, therefore^ best 

 proved, by directing it to the stars: if it shews, clearly, the fascix, on the 

 disk of the planet Jupiter, or the ring of Saturn, it will deserve to be ap- 

 proved, of. I have ground and polished, in the manner here described, the 

 mirrors of a little Gregorian telescope, of nine inches focus, which shewed 

 these objects most distinctly; and I could not afterward, with much greater 

 pains, execute another one, (neither indeed did I ever see one,) of that size, of 

 equal accuracy: which served to convince me, of the exquisite correctness re- 

 quired in the figure of the mirrors, and of how great perfection the instrument 

 is susceptible. Telescopes have been recommended, for their enabling persons 

 to read gilded letters, at a considerable distance: but this is an improper 

 method for determining their merits; for (beside the ground of error now 

 mentioned) a much greater quantity of light is reflected from gilt letters, than 

 from those of common print on paper. 



