170 



either of these, the tays, which issue in a cone or pen- 

 cil, from single, luminous, distinct points, in a very remote 

 object, and fall on them, will iiot converge again, to so 

 many single points; but will, in the mean focus of the mir- 

 ror, be dispersed, and blended together in a small de- 

 gree, yet sufficient to produce an universal haziness and 

 indistinctness, over the whole surface of the object view- 

 ed in a telescope, having its large mirror of these forms, 

 because it occurs, with respect to every point in such 

 object; of which the following are the circumstances. 



If the mirror be spherical, those, rays, nearly parallel 

 of each pencil, which fall on it, next to its center, will 

 converge to a point more distant from the mirror, than 

 the focus of any rays, that fall between the center and 

 outer edge of the mirror. And those, that fall on the 

 outer extremity of it, will converge to a different point, 

 hearest to the mirror: and the rays, which are incident 

 on the several concentrical annuli, indefinitely narrow, 

 of which the foce of the mirror is composed, will have 

 an indefinite number of points of convergence; each an- 

 nulus its own point, and all lying in a series, in the axis 

 of the pencil, between the points, or foci, of the extreme, 

 and of the innermost annulus.* So that no entire incident 



.penC|il 



* This property of a spherical mirror has never, so far as I know, been 

 synthetically ilemonstrated, by any optic writer, though it is a funditmental 

 theorem in catoptrics. Mr, Robins derisively objected to Dr. Smith, that he 



had 



