of Metals for refteBing felef copes. 337 
inftrument. All thefe pieces therefore together will 
completely Hint up the mouth of the telefcope. 
Let the round piece which covers the center of the 
metal, or that which has no hole in it, be removed; and, 
by a nice adjuftment of the fcrew, let the image (which 
is now formed by the center of the mirror) be made 
as iharp and diftindt as poffible. This being done, 
every thing elfe remaining at reft, replace the central 
piece, and remove the outfide annular one, by which 
means the circumference only of the fpeculum will be 
expofed, and the image now formed will be from the 
rays reflected from the outfide of the metal. If there be 
no occafion to move the fcrew and little metal, and the 
two images formed by thefe two portions of the metal 
be perfectly fharp and equally diftindt, the fpeculum is 
perfedt, and of the true parabolic curve; or at leaft the 
errors of the great and little fpeculum, if there be any, 
are corredted by each other. 
If, on the contrary, under the laft circumftance, the 
image from the outfide of the metal fhould not be dif- 
tindt, and it fhould become neceflary, in order to make it 
fo, that the little fpeculum be brought nearer, it is plain 
that the metal is not yet brought to the parabolic figure ; 
but if, on the other hand, in order to procure diftindtnefs, 
you be obliged to move the little fpeculum farther oft', then 
the figure of the great fpeculum has been carried beyond 
Vol. LXVII. Xx the 
