184 



oi\ a piece of looking-glass-plate, by a piece of the like 

 olass luiving a handle of brass cemented to it. This fur- 

 nished a very fine and impalpable powder, capable of 

 communicating to the specula, or lenses, the most exqui- 

 site polish and lustre.* 



To apply with precision, and afford a fair trial of the 

 method of polishing I have recommended, it is neces- 

 sary farther to consider, that the advantages, resulting 

 from correctness of figure, in the mirrors, may be frus- 

 trated, by an undue position of them, or of the lenses, iu 

 the instrument, or by a defect of form in the lenses, whose 

 edges may happen to be thicker on one side than on the 

 other; i. e. they may not be complete, or equally curtate 

 segments of spheres ; and, consequently, that a proper trial 

 and estimate cannot be made, of the figure of the mirrors, 

 unless these and the eye-glasses be right in these respects; 

 especially in the Gregorian telescope, whose adjustment is 

 a matter of more nicety and difficulty, than that of the 

 Newtonian. And since, in the former, the surfaces of the 

 mirrors and lenses ought all to have one and the same 

 axis, viz. that of the instrument, in which are to be all 

 their foci; it is necessary this should be cautiously ascer- 

 tained: because contrary deviations of them, in this re- 

 spect, might apparently compensate one another, and es- 

 cape detection, though they would really be attended with 



the aberrations of enlarged apertures. 



The 



* The same powder, spreai\ on leather, would give the smoothest edge to 

 razors and lancets, &,c. 



