54 REFLECTING TELESCOPE. 



thus be adapted for communicating the same figure to the 

 mirror. 



I have been led to adopt and practise this method of po-» 

 lishing mirrors, by the train of reflections and reasoning 

 herein described , and with sufficient success, for its unre-; 

 served recommendation. In one particular, it corresponds 

 >\'ith the method published by Mr. Mudge, in the Philoso- 

 phical Transactions, viz. in the direction of the motion used 

 in polishing the mirror. But this seems to have been pre- 

 scribed by him, without any respect to the properties of 

 mobility and inequality of friction, in the pitchy coating of 

 the polisher ; which things he has not noticed. And yet, as 

 any sort of motion, without a proper regard and adaptation 

 to the qualities of the pitch, would be ineffectual, it is here 

 attempted to supply that defect ; because no method can be 

 rightly pursued in practice, nor its success be uniform, nor any 

 figure already given to the mirror be altered, if those artists, 

 who would follow it, are ignorant of the principles and 

 agency on which it is really founded. For, in every process 

 of so subtil and delicate a nature, some untoward accidents 

 and circumstances must occur, which will grow above the 

 control and correction of any person, who is not aware of the 

 secret cau?es from whence they arise. In such cases, the 

 practice v/ill be as imperfect as the theory is. 

 Method of tak- It has been above explained, how the middle zone, or tract 

 the^ace^oAhe ^^ ^^^ polisher, equidistant from its inner and outer edge, 

 polisher. when there is a void at the center, will oppose a greater de-? 



gree of friction to the mirror, than the other parts of the 

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

 ror, by the increased action of this zone, it will be proper, 

 that, agreeable to the methods of preventioTi of this effect 

 before mentioned, there should be circular furrows indented 

 in the pitch within this zone, more or fewer, according to the 

 s\ze 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 may be diminished. 

 This will be very easily accomplished, by putting, the po- 

 lisher on the arbor of a lathe, and cutting out some of the 

 pitch in circular grooves, with a small sharp and concave 

 turning chisel, wetted with water, in which some soap has 

 been dissolved. And this may be performed and repeated. 



