REFLECTING TELESCOPE. 57 



kind of motion be used, in bringing the foce of the mirror to a 

 polish, the parabolic form is directed to be acquired, only by 

 a circular motion in polishing: Mr. Mudge having declared, 

 that the effect of such straight strokes would be, to produce no 

 other than a correct spherical figure in the mirror. Here, then, 

 are opposite motions, and declared to be productive of con- 

 trary effects, proposed by two very intelligent artists, with a 

 view of promoting the same effect; the only difference being 

 this, that, in the one case, the face of the polisher Is supposed 

 to be round, and in the other, oval : a difference that a person 

 may well imagine to be (as it really is) of very little impor- 

 tance ; and he may be easily led to suspect, that the presumed 

 effect of either mode is only imaginary ; that a spherical figure 

 of the mirror has been mistaken for a parabolical one : or that, 

 if the latter has been produced, it may have been, not by 

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

 rule or method advanced for this purpose. Thus, when dif- 

 ferent instructions are given, by different persons, without 

 any reasons or explanations assigned as the foundation of them, 

 the whole rests on authority ; authorities clash, and then the 

 worst may be followed, or all be rejected ; and, for want of a 

 guide, an uncertain practice be adopted. It is for this reason, 

 I have judged it necessary here, (as also in former essays, made 

 pubhc,) to be very minute, in attempting to investigate the 

 grounds of any method to be pursued, and the principles of ac- 

 tion, in the operation of the instruments I am treating of. 



I have made a trial of the method of polishing, proposed by commcnda- 

 Mr. Edwards, with attention to all the circumstances, which he tion ef the me- 

 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 correct a figure of the mirror, as, perhaps, 

 any other, yet made public. But, whoever will attentively in- 

 vestigate the nature of the operation, will, I think, cease to 

 wonder, that modes of conducting it, seemingly so dissimilar 

 tend to the same effect , and perceive, that the contrariety is 

 not real, but merely apparent *. 



For, in either method, it is not the direction of the motion 



* In the methods of figuring the mirrors, published by Mr. 

 Mudge, and by Mr. Edwards, it is stated by Mr. Mudge, that he 

 frequently, during the process, applied to the polisher a concave 



