174 



of the little mirror, ought to be spherical or excessive; 

 and, if the great mirror be spherical, the other must be 

 parabolical or hyperbolical, according as its focus is long 

 or short, in respect of that, of tlie gi'eat mirror. 



Should the telescope be faulty, from indistinctness 

 of vision, it may be coiTccted, by altering the figure of 

 either of the mirrors, as shall be most practicable. 

 And, to know what the alteration should be, the me- 

 thod, directed by Mr. JNIudge, may be followed, of ex- 

 eludino; the ligrht from the centra^ middle, or extreme 

 zone of the great mirror, by fixing, on the mouth of the 

 tube, three annular diaphragms of pasteboard &c., an- 

 swering, in size and shape, to these zones respectively; by 

 removing any of M'hich diaphragms, the light will be ad- 

 mitted to the corresponding part of the mirror. If, then, 

 by help' of the adjusting screw, the object be first viewed 

 distinctly, when the inner or central zone, only, of the 

 mirror is uncovered to the light; aiid it be necessary, af- 

 terward, 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 de- 

 ficient 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 sphe- 

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

 ft)cus of. the oiitei. zone of the little mirror is nearer to 



the 



