100 Mr. Samuel Hunter, 



If proper care were taken, the surface was now free from 

 scratches, capable of reflecting a considerable amount of light, 

 and of a spherical form. The polishing now commenced, the 

 object of which is not only to make the surface reflect the greatest 

 amount of light, but also to give the parabolic figure, and in this 

 consists the great difficulty. 



Speculum metal is very porous ; a moderate magnifying power 

 will show the surface full of little holes, so that if we used an 

 elastic cushion of any kind to carry the polishing powder, the 

 result would be that the powder would be forced into these pores, 

 whose edges would be worn away, and the entire surface would 

 thus be full of little pits, so that a true figure or good reflecting 

 surface would be impossible. 



On the other hand, if we used a hard unyielding substance, we 

 could only produce a spherical surface, as it is by the mutual 

 adaptation of the two surfaces that we are enabled to obtain the 

 parabolic. 



Fortunately, in pitch we have an almost inelastic body capable 

 of adapting itself to any surface with which it is in contact, and 

 yet being a solid. Ice is the only other solid that I know of 

 which possesses this latter property. The tool used in grinding, 

 or another made to the same curvature, usually a little larger 

 than the speculum, was now covered with pitch to a depth 

 varying from the thickness of a half-crown to half an inch ; 

 grooves were cut in this to allow it to expand equally in a 

 lateral direction, and thus become more quickly adapted to the 

 speculum. Some made the polisher smaller than the speculum, 

 and others used an oval polisher. 



The Rev. Mr. Edwardes, writing in 1787, recommends that the 

 longest diameter be to the shortest as 10 : 9, the latter being 

 equal to that of the speculum, and seems to be the first who used 

 the oval polisher. Sir John Herschel speaks highly of the oval 

 form, as also Messrs. Delarue and Nasmyth. 



The consistency of the pitch should be such that a sovereign 

 resting on its edge for one minute should leave clear impressions 

 of three of the millings on its edge (Lassell's test) ; if softer than 

 this it will not produce a good figure, if harder an inferior polish 

 will be the result. The pitch can be made harder by boiling or 

 softer by the addition of oil of turpentine. 



