56 REFLECTING TELESCOrE. 



in the polisher. And I believe it is on this account, calher 

 than that of preventing aberrations of the rays of light, from a 

 supposed spherical shape of Ihe mirrors^ that telescopes of 

 greater apertures and foci are more accurate ; the larger sur* 

 faces of their mirrors having a tendency, during the operation 

 of polishing, to preserve the regularity of their figure. For, 

 let the aperture of a telescope be ever so large, with respect 

 to the focus of the great mirror ; yet when the object is very 

 /. larger spe- remote, the central part of the field of view (the rays of light 

 eSr figured. ^^^^ which are parallel to the axis,) ought to appear perfectly 

 distinct, if the metals were wrought up to the correct figure 

 of conoids : and the vulgar doctrine of aberrations, which 

 relate only to spheres, is entirely inapplicable. The only 

 standard, for the measures of the apertures and foci, is the 

 degree of ingenuity in the Avorkman, who fabricates the in- 

 strument. There are many defects in figure, besides a sphe- 

 rical form of the mirrors ; and it happens but too frequently, 

 that a telescope is very indistinct, from a bad figure of them, 

 though that figure is the nearest to a conoid of" any regular 

 curve : for this is often the case, when the central, the ex- 

 treme, and the intermediate parts of the mirror, successively 

 and separately exposed to receive the light from the object, 

 appear to have the same focus. And this mostly occurs, when 

 the mirrors are small ; certain tracts, or portions of their sur- 

 face, being more worn down, by the grinding or polishing, 

 than others, arising from the difficulty of preserving an uniform 

 pressure during the operation, and, consequently, a regular 

 figure of the polisher. 



Another method, diflTerent from that now described, g€ 

 communicating to mirrors a parabolic form, has been discove- 

 red by the late Rev. M. Edwards, and published in the Nau- 

 tical Almanack, for the year 1787. He recommends, <o make 

 the edge of the polisher the periphery of an ellipse; so that tjie 

 •face or area of it may not be round, but oval : the shortest 

 diameter of the ellipse being equal to that of the mirror j and 

 its longest diameter to be to the shortest, as 10 to 9. And 

 he affirms, that a miri'or, finished on such a polisher, will prove 

 to be of a parabolic form; if the process be conducted, by 

 employing, throughout the operation, straight strokes of the 

 mirror, diametrically acros the polisher, in every direction. 

 Now in the method recommended by M. Mudge, whatever 



