84 



REFL'ECTING TELESCOPE* 



The polisher 

 must be of 

 pitch, rather 

 joft. 



III. 



Observations on the Metallic Composition of the Specula of 

 reflecting Telescopes, and the manner of casting them ; 

 also a method of communicating to them any particular 

 Conoidal Figure ; loith an attempt to explain, on scientific 

 Principles, the Grounds of each Process, and occasional 

 Remarks on the Construction of Telescopes. By the Rev. 

 James Little*. 



[Conchided from p. 59.] 



T, 



HE consistence of the pitch is, in this business, an 

 article of the first importance. Soft pitch will give to ths 

 polish a higher lustre, and will less expose the face of the 

 mirror to scratches : but, if it be too soft, the mirror will 

 sink in it, like a seal in soft wax; and the figure of the 

 polisher cannot be preserved, nor the furrows in it from 

 being defaced. It must, therefore, be always harder than 

 common pitch is, in a mean temperature of the air in this 

 climate. And, after the polishing powder is bedded in it 

 (which must at first be laid on so copiously, that the pitch 

 may not rise up to the surface, between the particles of it,) 

 and when the mirror has been worked on it a little time, 

 then all the loose particles of the powder ought to be washed 

 off, from the edges and furrows of the pitch, with a sponge , 

 or brush, (made of fine hair,) under water, that no grains 

 may get on the surface, and injure the polish. And, if this 

 be attended to, and the pitch be a little softened by heat, 

 when the powder is first applied, it may be used of a con- 

 sistence hard enough, M'ithout inconvenience : butj^ if it bo 

 made so hard, as not to sink at all, or expand itself, under 

 the mirror, I believe it will never communicate to it a perfect 



The Urger the 

 j)olisher the 

 flatter the 

 figure. 



From what has been here laid down, it must be obvious, 

 that, by diminishing the size of the polisher, whether it be 

 of a circular or elliptic shape, the curvature of the mirror 

 will be brought nearer to that of a circle; and, by enlarging 

 the polisher, the curvature will approach to that of an hy~ 



Irish Transactions, Vol. X. 



perbola, 



