169 



Hence, the excellence of an elliptic figure, for the smaller 

 speculum of the Gregorian telescope.* But the case is 

 very different, in a spherical or hyperbolic mirror. From 



either 



* But this will not be the case, if the rays diverge from points, so remote 

 from the axis of this speculum, as to make a considerable angle with it, and 

 to fall very obliquely on the speculum : which would be the case, if the field 

 of the telescope were too large, or if the focus of the great speculum were 

 too long, with respect to that of the lesser concave 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 latitude; and the extreme pencils, diverging 

 from it, fall, with too much obliquity, on the latter, to be collected by it, to 

 single points, in the second image. And, on this account, there is, in the 

 Gregorian telescope, 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 aberra^ 

 tions 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. 1 am of opinion, that it is better not to aim at, 

 a high degree of magnifying, by the little mirror of this telescope ; but, to 

 endeavour to secure the correctness of the second image ; and to lay the chief 

 stress of the amplification (as it is in the Newtonian telescope) on the eye- 

 glasses; because of the above circumstance, which no correctness, nor com-* 

 pensation of the mirrors, can remedy. From this inconvenience, here stated,, 

 the Newtonian telescope (the most perfect of all the constructions, that ever 

 ■were or ever will be devised) is entirely free. This the author effected, by 

 putting 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 ap- 

 pear, on due consideration, that he was obliged to introduce this change, in 

 Gregory's telescope, of necessity; and not from a low ambition, 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 form,, nor the advantages of his own; having only, in an- 

 swer to objections, and, as it were, reluctantly, mentioned the chief circum- 

 stances, justifying the alteration he had recommended. 



