160 



is supposed to be round, and in the other, oval: a dif- 

 ference that a person may well imagine to be (as it 

 really is) of very little importance; and Ire may be easily 

 led to suspect, that the presumed effect of either mode 

 h only imaginary; that a spherical figure of the mijTor 

 has been mistaken for a parabolical one; or that, if the 

 latter has been produced, it may have been, not by me- 

 thod, but by chance; and he may naturally distrust any 

 rule or method advanced for this purpose. Thus, when 

 different instructions are given, by different persons, with- 

 out any reasons or explanation assigned as the foundation 

 of them, the whole rests on authority; authorities clash, 

 and then the Avorst may be followed, or all be rejected; 

 and, for want of a guide, an uncertain practice be adopt- 

 ed. It is for this reason, I have judged it necessary here^ 

 (as also in former essays, made public,) to be very mi- 

 nute, in attempting to investigate the grounds of any 

 method to be pursued, and the principles of action, in 

 the operation of the instruments I am treating of. 



I have made a trial of the method of polishing, pro- 

 posed by Mr. Edwards, Avith attention to all the circum- 

 stances, which he directs to be observed; and, from the 

 result, I have reason to believe, that his method is a 

 good one, and will, if judiciously applied, produce as cor- 

 rect a figure of the mirror, as, perhaps, any other, yet 

 made pub|ic. But, whoever will attentively investigate 

 the nature of the operation, will, I think, ce*ase to won- 

 der, 



