pears, that, to make the pitch too hard and refractory, 

 would be to destroy every property in it; which renders 

 it eligible in this operation. 



If the positions, before stated, be well founded, it seems 

 to follow, that the desired change in the mirror, from a 

 spherical to a conoidal figure, can only be eifected, by a 

 change in the shape of the polisher, graduall}' accommo- 

 dating itself to the alteration, produced in that of the 

 mirror, during the process of polishiijig. Nor, indeed, can 

 it well be conceived, how the mirror, could alter its sphe- 

 rical form, if that of the polisher remained imaltered; 

 for a conoid could never, in the usual way, and Avithout a 

 partial separation of the surfaces in contact, be polished 

 on a segment of a sphere, nor even on that of a conoid, 

 if, during the friction of their surfaces, the center, or ver- 

 tex of the one, weie to be moved to any considerable dis- 

 tance from that of the other. So that the strokes, in po- 

 lishing, must never ultimately be earned so far, as to re- 

 move the centei; of the mirror to too great a distapce! 

 from that of the. polisher; even though its surface wqre so 

 hard, as to preserve its figure unaltered by the pressure' 

 of the mirror.* ■ ""' 



•u 2 ,. Agreeabk- 



■ o) ^'■><^'y^'^ ■;; : :•? 



* For the several reasons above-mentioned, I am inclined to think, it iwil! 

 be very difficult to discover a method, different from that here explained, o^' 

 communicating, at the same time, a perfect figure and: polish to a speculum. 

 It is plain, that Newton could think of no better; though I imagine that, 

 in this instance, he tried his inventive powers with those of Des Cartes, who 

 had pubhshed a method (in theory elegantly geometrical) of figuring optic 



glasses. 



