176 



■'Now, the circumstances, \vhich,''' in tlie method of po- 

 /ishing above-mentioned, have a tendency to produce par- 

 ticular species of conoids, have been ah'cady explained, 

 and need not be repeated. But, as to the means of alter- 

 ing any. figure already given, to the great mirror, in the 

 Newtonian telescope, or to either of the mirrors in the 

 Gregorian, which happens to be unsuitable to the other 

 one; I have to observe, that, in my trials, I have found 

 this could be effected on the polisher, without putting the 

 hietal to be ground again upon the hones. For if it has, 

 at first, been formed to a tolerably correct figure, of any 

 species, then a very small reduction, of the substance of 

 the metal, will produce a sufficient alteration of its form. 

 If the change required consists in a diminution of cur\-a- 

 tiird;' a continuation of the process, imder the regulation 

 before-mentioned, will, without any alteration of the po- 

 lisher, generally, be sufficient to produce it, from the de- 

 gree of curvature of a circle, to that of the ellipse, pa- 

 rabola, or hyperbola, in order; or from any of these, to 

 the others, in succession.* But, if the degree of curvature, 



already 



* Here it may be proper to observe, that, as the curvatuie is constantly 

 diminishing, by the mere continuance of the operation, so the process is not 

 to be pursued any longer, after the polish, and tlie desired figure, are found 

 to be perfected. And the metal must always be brought to a very fine face, 

 and a correct spherical figure, on the hones, or on a leaden tool, bedded with 

 the finest washed emery, before the process of polishing commences: because, 

 if all scratches, from the grinding, be not previously obliterated, the polishing 



must. 



