2J8 SUPPLEMENT TO THE 



we shall suppose that its figure has been truly generated 

 by the revolution of some conic section, but that it has 

 afterwards been rendered unequal in different places, either 

 in the polishing, from being kept too long on the tool, or 

 made so on purpose, by abrading its surface in different 

 places. In this case the contour of the disc is irregular, 

 and the annuli correspond with it, but generally become 

 truer as they approach the lucid point at the centre. But 

 the reader must recollect, that, when a metal operates 

 along with another, there will be a shadow of the small 

 one, and its arm projected on the disc, as at figure 8 /x , 

 which represents the appearance of the Amician reflecting 

 engiscope, shewing an artificial star out of focus : the same 

 observation will apply to all discs generated by reflecting 

 instruments. 



By observing what part of the metal it is necessary to 

 cover, in order to obliterate the image of some irregular 

 part of it, the identical portion of its surface which is in 

 fault may be discovered and amended. 



These irregularities are chiefly observable in object- 

 glasses and metals of small angles of aperture. It would 

 be useless to specify all the varieties of error which may 

 occur; but an oval disc is perhaps the most common. 



Sometimes, in the process of working, two or more 

 curves are generated in place of one, or it may be several 

 curves of the same figure and focus which do not bear 

 upon one focal point; in this case, the metal or object- 

 glass shews two or more discs, which do not lap over each 

 other, as at figure 12, and in this way give a false contour, 

 but which may always be discriminated from figure 11, by 

 observing the direction of the annuli, and the outline of 

 the overlapping discs. 



