44- keflecting telescope. 



Remedies. ^^^ ^^^^^ inconveniences, however, arising from the un- 



equal friction of the polisher, there are the following easy 

 and adequate remedies • which will, in the sequel, be more 

 fully explained, and applied as in practice, to effect the degree 

 of curvature, or any correction of the same, which may be 

 requisite. 

 Make the po- Fir;t. Since the curvature of tlie mirror ought to be gra- 

 lislicr larger dually reduced towards its edges, which can only be effected 

 than the mir- , . r r • i.- • ^l j- ^ r ^i 



jor by an increase ot friction ni the corresponding part of the 



polisher ; and that this latter effect is to be produced in any 

 part of it, by enlarging the surrounding coat of pitch : it fol- 

 lows, that, for this purpose, the brqadth of the pohsher must 

 be enlarged above that of the mirror; and this in the same 

 degree, as the curvature of the mirror is. to be diminished : so 

 that the polisher is to be of greatest breadth, for a mirror 

 of an hyperbolic, and least, for one of a spherical figure. 

 This, however, is to be done, under the limitations hereafter 

 mentioned. 

 Contract the Secondly. To preserve the regular gradation of curvatur© 

 ccn er lo e. towards the middle of the mirror, the uncoated space, at the 

 center of the polisher, should be contracted to a certain limit, 

 which will be defined ; though, for the reasons above men- 

 tioned, it can never be filled up altogether^ 

 Cut out some Thirdly. Where the resistance and friction of the pitch, 

 of the face of in any tract on the lace of the polisher, is computed as above, 



the pchsher ^^ found in effect, to be too great ; it may be lessened and 

 wlitrc the re- ' & ' / 



action is great- regulated, in any degree, by cutting, 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 

 ofpitch directly, 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 cavities, the con- 

 tinuity of the pitch being dissolved, its resistance, depending 

 thereon, may be modified at pleasure. 

 In this way, I" ^^"s manner may the polisher be so disposed, as to com- 



thc small, as municate a correct figure to large mirrors, and even to those 

 lar^-^e^eru- of smallest size. Now, whatever success may have attended 

 lums, may be the efforts of other persons, in communicating a proper figure 

 4uly figured. ^^ ^^^ g^^^^ speculum, (especially Mr. Short, vjrhom I have 

 manifold reasons for believing to have been among the most 



