REFLECTING TELESCOPE. g^ 



If the mirror be of an elliptic, or oval, curvature, rays, The small spe- 

 issuing Irom single points, in, or extremely near to one of culumoughtto 

 its foci, and falling on it, (such as the rays proceeding from ^^«"'P^^*^»^ 

 the single points in the image, formed by the parabolic larger 

 syeculum,) will be converged to so many single distinct 

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

 the excellence of an elliptic figure, for the smaller speculum 

 of the Gregorian telescope *. But the case is very different, 



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

 IS susceptible. Telescopes have been recommended, for their enabline 

 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. 



* But this will not be the case, if the rays diverge from points, so t. • i • u, 

 remote from the axis of the speculum, as to make a considerable angle tLautTafnU 

 With It, and to fall very obliquely on the speculum : which would be fying power of 

 the case, if the field of the telescope were too large, or if the focus of ^^^ Gregorian 

 the great speculum were too long, with respect to that of the lesser t^elescope 

 one: because. In either case, the image, formed by the great mirror ''^°'^},'^ ^^ , 

 which Is the object, with regard to the lesser, will have too great lad ' enl' J^T"^' 

 tude; and the extreme pencils, diverging from it, fall, with ' oo Ju h pkce ^ 



obhquity,on the latter, to be collected by it, to single points in the 

 second image. And, on this account, there is, in the Gregorian tele- 

 scope, a limit set to the degree of magnifying, so far as this depends on 

 the mirrors, be their figure ever so correct. And, if any aberration 

 prevail, in the image formed by the larger concave, they will be mag- 

 nified by the lesser, were it perfect, in the proportion of the focus of 

 the former to that of the latter. I am of opinion, that It is better not ' 

 to aim at a high degree of magnifying, by the little mirror of this tele* 

 scope; but, to endeavour to secure the correctness of the second 

 image; and to lay the chief stress of the amphfication (as it is in the 

 Newtonian telescope) on the eye glasses ; because of the above circum- 

 stance, which no correctness, nor compensation of the mirrors can 

 remedy. From this inconvenience, here stated, the Newtonian tele- 

 »cope (the most perfect of all the constructions, that ever were or ever 

 will be devised) Is entirely free. This the author effected, by puttlne 

 the eye-glasses on a different axis from that of the mirrors; by which 

 he was enabled to make the lesser mirror a plane surface. And it will 

 appear, on due consideration, that he was obliged to introduce this 

 change, in Gregory's telescope, of necessity; and not from a low am- 

 bition, to which his mind was superior, that of obtruding his own 

 inventions, to supplant those of equal merit by other men : "though he 

 has not stated all the imperfections of the Gregorian forifi, nor the 

 advantages of his own; having only, In answer to objections', and, as 

 it were, reluctantly, mentioned the chief circumstances, justifying 'the 

 alteration he had recommended. 



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