158 



motive force bear to the force of cohesion, which tends to 

 preserve an uniformity of pressure in the mirror, and of 

 figure in the poUsher. And I believe it is on this account, 

 rather than that of preventing aberrations of the rays of 

 light, from a supposed spherical shape of the mirroi-s, that 

 telescopes of greater apertures and foci are more accurate; 

 the larger surfaces of their mirrors having a tendency, dur- 

 ing 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 remote, the central part of the 

 field of view (the rays of light from which fall 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 sphereSj, 

 is entirely inapplicable. The only standard, for the mea- 

 sures of the apertures "and foci, is the degree of ingenuity 

 in the workman, who fabricates the instrument. There are 

 many defects in figure, beside a spherical form of the mir- 

 rors; 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 extreme, and 

 the intermediate parts of the mirror, successively and se- 

 parately exposed to receive the light from the object, ap- 

 pear to have the same focus. And this mostly occui-s, when 

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

 surface, being more worn down, by the grinding or pohsh- 



