REFLECTING TELESCOPE. 5P 



destruction of ray house,) to prosecute the experiments, which, 

 plight have enabled me to speak with more precision ; and 

 which I would have done, from the desire I had, to contribute 

 to the perfection of so noble an instrument as the reflecting te^ 

 lescopcp 



I know, that both methods wiH^ injudicious practice, pro- Both may sue 

 duce the desired effect ; but this effect will be limited, in de- 

 gree of perfection, and sometimes frustrated, when the causes 

 and circumstances, that operate in it, are unknown. In either 

 method, and with a polisher of round or oval shape, it is indis- 

 pensably necessary, that there should be furrows made in the 

 coating of pitch, (to allow it to subside, in regular gradation, 

 from the middle to the edges,) by indenting it, either in squares, 

 as is usually done, or in circular channels; both which mast 

 be renewed, as they become filled up and obliterated ; which 

 will always happen soonest in the middle zone or tract of the 

 polisher, between the center and other edge, whether the fur- 

 rows be circular or longitudinal: and, if this be not done, the 

 regularity of curvature would not be preserved in the mirror, or 

 the polisher. But, since there is no obstacle to the subsidence 

 of the pitch, near its outer edge, and its inner edge, when 

 there is a void space at the center, I believe the furrows 

 ought not to be made there, but in the intermediate space 

 only. And I am of opinion that it is, from the judicious dis- —tut these 

 position of these furrows, the most correct shape of the mirror ^^^^ be fur- 

 is to be acquired, whether the polisher be round or oval, or pitch. 

 the pitch hard or soft : for I found, that, in Mr. Edwards's me- 

 thod, and with pitch, even as hard as he recommends, the 

 channels made in it were, towards the end of the operation, 

 nearly obliterated, in the middle zone of the polisher. But this 

 will not happen so soon, nor so dangerously, with hard as 

 with soft pitch ; nor will the correction of the impaired shape 

 of the polisher be so difficult, when it is of an oval, as when of 

 a circular area : there being, in the former case, less of irregu- 

 lar surface in it, to be reduced ; and a more steady, uniform, 

 and simple motion, in grinding, may be pursued; which, as it 

 will admit of a less degree of expertness and sagacity in the ar- 

 tificer, will be more commonly attended with eminent suc- 

 cess*. (To be Continued.) 



* I imagine, that a polisher, whose area is of an oval form, 

 would be better adapted to the formation of a parabolic, than an 



