165 



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

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

 in this cHmate. And, after the poUshing 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 consistence hard enough, without in- 

 convenience: but, if it be 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 figure. 



Fi'om 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- 

 perbola, when the precautions here given are observed. 

 Both these may be done, by spreading the pitch on the 

 polisher, to a greater or lesser extent. 



In the Gregorian telescope, the excess of curvature, in 

 the great mirror, may be remedied, by a defect of it in 

 the little mirror, and vice vers^. It must be desirable, to 

 a fabricator of this instrument, to understand why this is 



VOL. X. Y so; 



