REFLECTING TELESCOPE. 91 



zone, only, of the mirror is uncovered to the light ; and it 

 be necessary, afterward, Avhen it is seen by means of the 

 exterior zone only, to remove the little mirror farther from 

 the great one, (by turning back the adjusting screw,) in 

 order to distinct vision; then one, or both of the mirrors, is 

 deficient in curvature, i. e. the great one is hyperbolical, or 

 the small one parabolical. And, on the contrary, if it be 

 necessary, in this process, to bring the little mirror nearer 

 to the great one ; then one or both of the mirrors is spheri- 

 cal. For, in the former case, it is plain, that the mean 

 focus of the outer zone of the little mirror is nearer to the 

 second eye-glass, than that of the inner zone ; since it is 

 necessary to withdraAV that focus, by putting back the little 

 mirror; and the contrary is evident, in the latter case. The 

 former could happen, only by the focus of the extreme rays, 

 of each single pencil, being too far from the great specu- 

 lum, (i. e. from its being hyperbolical,) and too near to the 

 little OQe ; or from the latter being deficient in curvature, 

 near its edges; and thus throwing the focus of the rays, 

 that fall there, too far from it, and too near to the last eye- 

 glass. The second effect could arise only from a figure of 

 the mirrors, the reverse of this. In the Newtonian tele- 

 scope, there can be no doubt, where the defect of curvature 

 is, because it has but one concave mirror. 



When it has been thus determined what the defect is, How to correct 

 means must be employed to correct it; and it may be ex- '^^ curvatures 

 pected, that, unless some certain mode, of eff'ecting a dif- 

 ferent curvature of the great mirror, from that of the little 

 one, is discovered, and skilfully practised, there will be 

 but few 'good telescopes, of the Gregorian form, constructed. 

 For, if both mirrors be polished, in the same manner and 

 method, it is likely, that the defects in their figure, and the 

 species of their curvature, will be the same in both. Where- 

 as, it has been shewn, that all these ought to be directly 

 contrary in one, from what they are in the other ; referring 

 to the parabola and ellipse, as the standard degrees of cur- 

 vature. 



Now, the circmnstances, which, in the method of polish- __by means of 

 ing above-montioiied, have a tendency to produce particular the polisher, 

 species of conoids, have been already explained, and need ^'^^°"^^""^" 

 not be repeated. But, as to the means of altering any figure '"° 

 H 2 already 



