346 Mr. mudge on the Conjl ruction 
As it is always neceffary to folder to the back of the 
little fpeculum a piece of brafs, as a fixture for the fcrew 
to adjuft its axis, I fhall juft hint a fafe and neat method 
of doing it, which may be very ufeful to the optical or 
mathematical inftrument-maker upon other occafions. 
Having cleaned the parts to be foldered very well, cut out 
a piece of tin-foil the exadt fize of them ; then dip a fea- 
ther into a pretty ftrong folution of Jal ammoniac in wa- 
ter, and rub it over the furfaces to be foldered; after 
which place the tin-foil between them as faft as you can 
(for the air will quickly corrode their furfaces fo as to 
prevent the folder taking) and give the whole a gradual 
and fufficient heat to melt the tin. If the joints to be 
foldered have been made very flat, they will not be 
thicker than a hair : though the furfaces be ever fo ex- 
tenfive, the foldering may be conducted in the fame man- 
ner, only that care muft be taken, by general preflure, to 
keep them clofe together. In this manner, for inftance, 
a filver graduated plate may be foldered on to the brafs 
limb of a quadrant, fo as not to be difcemable by any 
thing but the different colour of the metals. This me- 
thod was communicated to me by the late Mr. jackson, 
who during his life kept it a fecret, as he ufed it in the 
conftrudtion of his quadrants, and is, I believe* not as yet 
known to any workman. 
In 
