VOL. LXVII.] PHILOSOPHICAL TRANSACTIONS. 177 



employ his attention. It was he who first proposed, and indeed practised, the 

 polishing with pitch ; a substance which at first sight perhaps every one but him- 

 self would have thought very improper, from its softness, to produce that cor- 

 rectness of figure so necessary on these occasions ; and yet I believe that it is the 

 only substance in nature that is perfectly well calculated for the purpose ; for at 

 the same time that it is soft enough to suffer the putty to lodge very freely on its 

 surface, and for that reason to give a most tender and delicate polish ; it is like- 

 wise totally inelastic, and therefore never, from that principle, suffers any altera- 

 tion in the figure you give it. If the first makers of the instrument therefore 

 had given proper credit to, or had simply followed the hint Sir Isaac gave, it 

 would have saved them infinite trouble, and they would have produced much 

 better instruments ; but the {)retended refinement, of drawing a tincture from 

 pitch with spirits of wine, affords you only the resinous, hard, and untractable 

 part of the pitch, divested of all that part of its original substance which is 

 necessary to give it that accommodating pliability in which its excellence consists. 



It is needless to swell this account with a detail of the process for polishing the 

 little speculum, as it must be conducted in the same manner which has been 

 already described in that of the large one ; only observing, that as the little 

 metal has an uninterrupted face, without a hole, so there is no occasion for one 

 in the polisher ; and also tliat, as a spherical figure is all that need here be prac- 

 tically attempted, so the difficulty in finishing is infinitely short of that of the 

 other. 



As it is always necessary to solder to the back of the little speculum a piece of 

 brass, as a fixture for the screw to adjust its axis, I shall just hint a safe and 

 neat method of doing it, which may be very useful to the optical or mathemati- 

 cal instrument maker on other occasions. Having cleaned the parts to be sol- 

 dered very well, cut out a piece of tin foil the exact size of them ; then dip a 

 feather into a pretty strong solution of sal ammoniac in water, and rub it over 

 the surfaces to be soldered ; after which place the tin foil between them as fast as 

 you can (for the air will quickly corrode their surfaces so as to prevent the solder 

 taking) and give the whole a gradual and sufficient heat to melt the tin. If the 

 joints to be soldered have been made very fiat, they will not be thicker than a 

 hair : though the surfaces be ever so extensive, the soldering may be conducted 

 in the same manner, only care must be taken, by general pressure, to keep them 

 close together. In this manner, for instance, a silver graduated plate may be sol- 

 dered on to the brass limb of a quadrant, so as not to be discernible by any thing 

 but the different colour of the metals. This method was communicated to me 

 by the late Mr. Jackson, who during his life kept it a secret, as he used it in the 

 construction of his quadrants, and is, I believe, not as yet known to any 

 workman. 



p. s. It was some time after I had written the above account tliat I saw Mr. 

 Short's method of polishing object glasses for refracting telescopes, published in 

 the Transactions. By that paper I find that what I before strongly suspected, 



VOL. XIV. A A 



