155 



center to the outer edge, in the form of a conoid, ancT 

 thus be adapted for communicating the same figure to the 

 mirror. 



I have been led to adopt and practise this method of 

 polishing mirrors-, by the train of reflections and reasoning 

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

 served reconamendation. In one particular^ it coiTesponds 

 with the method- published by Mr. Mudge, iuthe! Philoso^ 

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

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

 scribed by hin>, without any respect to the properties of 

 mobility and inequality^ of friction, in the pitchy coating 

 of the poHsher; which things he has not noticed. And yet, 

 as any sort of motion, without a- proper, regaixi and adap^ 

 tation tOi 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 unifonu, nor any figure already given to the mirror be 

 altered, if those artists, wIk) Avould follow it, are ignorant 

 of the principles and agency on which it is really founded". 

 For, iu'^ every process of so subtit 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 causes from whence 

 they . arise. . In such cases, the practicewill.be as imperfect 

 as the theory is. 



It has been above explained, how the ifiiddle zone, or 

 tract of the polisher, equidistant from its inner and outer 

 . :, edge. 



