90 ItETLECTING TELESGOtE. 



by it, to the same point with those of the other annuli, un- 

 less the curvature of the corresponding annulus, of the little ' 

 mirror, be increased, in the proportion of its radius to that 

 of the great mirror ; for, then only Avill the focus of rays, 

 reflected by this annulus of the little mirror, be shortened, 

 as much as, by the effect of that of the same annulus, of the 

 great mirror, it would otherwise be lengthened. The same 

 is true, viceversu^ if the focus of any annulus, of the great 

 mirror, be shorter than those of the other annuli. 



Fourthly. That, if there be any excess or defect, in the 

 curvature of the great mirror, from that of a parabola, (and, 

 consequently, a contraction, or elongation, of the foci, of 

 the extreme rays of the reflected pencils,) there is no remedy, 

 while this remains, but to make the little mirror so much 

 deficient, or excessive in curvature, from that of an ellipse, 

 (and, consequently, to lengthen or contract the foci of th« 

 extreme rays of the pencils reflected by it,) as its focus is, 

 in proportion to the focus of the great mirror 5 there being 

 no other means of reducing all the rays, of each pencil, to 

 one point, at the second or conjugate focus of the little 

 mirror; by which alone, the second image, consisting of 

 such points, can be formed, and viewed distinctly, through 

 the last eye-glass. 



From all which, it is manifest, that, if the curvature of 

 the great mirror be hyperbolical or deficient, then that of 

 the little mirror ought to be spherical or excessive ; and if 

 the great mirror be spherical, tiic other must be parabolical 

 or hyperbolical, according as its focus is long or short, in 

 respect of that of the great mirror. 

 Instructions Should the telescope be faulty, from indistinctness of 



for examining vision, it may be corrected, by altering the figure of either 



the adaptation ^^j^ mirrors, as shall be most [)racticablc. And, to know 



of the mirrors " ' ' 



to each other, what the alteration should be, the method, directed by Mr. 



Mudgc, may be followed,, of excluding the light from the 



' central, middle, or extreme zone of the great mirror, hy 



fixing, on the mouth of the tube, three annular diaphragms 



of pasteboard, &c. answering, in size and sha])e, to these 



zones respectively ; by removing any of which diapliragms, 



the light will be admitted to the corresponding part of the 



mirror. If, then, by help of the adjusting screw, the ob- 



ject be first viewed distinctly, when the inner or central 



zone. 



