156 



edge, when there is> a void at the center, will oppose a 

 greater degree of friction to the mirror, than the other parts 

 of the polisher. And, to prevent the unequal wearing of the 

 mirror, bj the increased action of this zone, it will be pro- 

 per," that, agreeable to the methods of prevention of this 

 effect before-mentioned, there should be circular furrows 

 indented in the pitch within this zone, more or fewer, ac- 

 cording to the size of the mirror, and the designed degree 

 of its curvature; in order that the pitch may subside into 

 the furrows, and thus the resistance and friction in that 

 tract be diminished. This will be very easily accom- 

 plished, by putting the polisher on the arbor of a lathe, 

 and cutting out some of the pitch in circular grooves, with 

 a small and sharp concave turning chisel, wetted with Avater, 

 in which some soap has been dissolved. And this may be 

 performed and repeated, if necessary, without any injury 

 to the surface of the polisher, if it be previously wetted, 

 to prevent the splinters of the pitch from sticking to it; 

 which may be washed off, by a soft brush or pencil, from 

 the polisher, it being inmiersed in water. 



Since, in the Gregorian telescope, the defect of figure 

 or curvature, from that of a conoid, in one of the mirrors, 

 may be compensated by a contrary curvature in the other; 

 and since, in either of the mirrors, whose breadth is given, 

 the degree of variation in its figure, from that of a sphere, 

 ought to be so much the greater, as its focus, or radius of 

 curvature, is shorter; it will, on this account, be far more 

 difficult, to effect a proper figure of the small mirror in 



this 



