141 



puted as above, or found in eftect, to be too great; it 

 may be lessened and regulated, in any degree, by cut- 

 ting, out of that part of its surface, some of the pitch, 

 at proper intervals, in narrow channels or furrows: the 

 number and depth of which ought to be proportioned to 

 their distance from the edges of the coat of pitch di- 

 rectly, and to the reduction of curvature, proper to the 

 corresponding parts of the mirror inversely, and should 

 be in a ratio compounded of both; for, by these cavi- 

 ties, the continuity of the pitch being dissolved, its re- 

 sistance, depending thereon, may be modified at pleasure. 



In this manner may the polisher be so disposed, as 

 to communicate a correct figure to large miiTors, and 

 even to those of smallest size. Now, whatever success 

 may have attended the efforts of other persons, in commu- 

 nicating a proper figure to the great spe<)ulum, (especially 

 Mr. Short, whom I have manifold reasons for believing to 

 have been among the most eminent opticians, as well 

 as artists, that have laboured in the improvement of this 

 instrument;) I have not heard, that any method has been 

 proposed, of communicating, to the little mirror of the 

 Gregorian telescope, any other than a spherical form, 

 which yet may in this manner be done. And it must, in 

 this telescope, be a thing most desirable to accomplish; 

 especially when its size and aperture is so great, that it 

 Wovdd be difficult to impress, on the extensive surface 

 of its great mirror, (merely by the small alteration of 

 figure, which could be produced, in the delicate opera- 



voi. X. T tion 



