scope. 



86 REFLECTING TELESCOPE. 



Newtonian or Poipt in the same, so very distant, that they may be regarded 

 Gregorian tele- as parallel, will converge, by reflection, to one point in the 

 axis ; which point is the focus. The same is nearly true of 

 rays coming from luminous points not far from the axis, or 

 lying in a very contracted field of view, so as to make but a 

 very small angle with the axis : the rays, coming from each 

 single distinct point in the object, are converged to so many 

 single distinct points in the image, formed at the focus of the 

 mirror. Hence, the excellence of a parabolic mirror, for 

 the larger speculum of the Newtonian or Gregorian tele- 

 scope*. 



If 



* Butjbecause a parabolic mirror reflects, to one point, rays, that 

 fall on it, parallel to its axis, it follows, that it will not converge, to a 

 point, rays, that are diverging or inclined to its axis. The former, (if 

 the point, from whence they radiate, be in the axis of the mirror,) 

 would be reflected from any line, drawn diametrically across the mir- 

 ror, in a caustic curve double and cuspidated: the latter, (being in the 

 same plane in which is the radiant pOlnt, infinitely distant, and the 

 axis,) would form a curve nodated. So that the excellence of a para- 

 bolic mirror is for viewing remote, but not near objects. And a per- 

 son might thus be deceived, who would judge of the goodness of a 

 telescope, only from its rendering print legible, at a small distance, 

 from whence the breadth of the great mirror would subtend an angle 

 of sensible magnitude: for the pencils of rays that issue, diverging 

 from each point of tbe 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 annull 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 ob- 

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

 zone, unless the little mirror were removed farther 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 tele- 

 scope is, therefore, best proved, by directing it to the stars: if it shews, 

 clearly, the fascijc, on the disk of the planet Jupiter, or the ring of 

 Saturn, it will deserve to be approved of. I have ground and polished, 

 in the manner here described, the mirrors of a little Gregorian tele- 

 scope, of nine inches focus, which shewed these objects most distinetly; 

 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 required in 



th? 



