REFLECTING TELtSCOPE. 



m 



peras, broken into grains as small as shot, by holding it 



over the firo, on a fire-shovel, till the moisture in it appeared 



to be dried away. I then put it into a crucible, and kept it 



uncovered, in a clear fire, till it had been some time red hot*; And le^^igatlng 



by which, the spirit or oil of the vitriol was distilled from ^^1°^^^^ ^''"^ 



it, and the calx or colcothar remained, of a brownish red 



colour, and of a perfectly equal texture, entirely free from: 



the hard or gritty particles ; and it was easily levigated, 



when moistened with water, on a piece of Iooking-gla,ss- 



plate, by a piece of the like glass having a handle of brass 



cemented to it. This furnished a very fine and impalpable 



powder, capable of communicating to the specula, or 



lenses, the most exquisite polish and lustre +. 



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

 method of polishing I have recommended, it is necessary ^"^^dTelLcopc 

 farther to consider, that the advantages, resulting from cor- if cither the 

 rectness of ficrurc, in the mirrors, may be frustrated, by an specula or the 

 .,. ° ' , f ., . • ^, . ^ ^ lenses bebadly 



undue position ot them, or or the lenses, xn the instrument, centered, or 



or by a defect of form in the lenses, Avhose edges may hap- not placed oa 

 pen to be thicker on one side than on the other; i.e. they t^e same axis, 

 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 Gre- 

 gorian telescope, whose adjustment is a matter of more 

 nicety aud difticulty, 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 ascertained; because contrary 

 deviations of them, in this respect, might apparently com- 

 pensate one another, and escape detection, though they 

 would really be attended with the aberrations of enlarged 

 apertures. 



* I suppose that the fire ought not to be too high, or too long con- 

 tinued in this process, lest it should convert the calx of the iron into 

 glassy scoria. Experiments will determine the due regulatJon of the 

 heat, so as to ensure success to the operation in every instance. The 

 heat ought to be so great, as to give the colcothar, not a brawtt, but a 

 red rolour. 



f The same powder, spread on leather, would give t;he smoothest 

 edgcto razors and lancets, &c. 



Vol. XVI.— Feb. 1807. I Th^ 



