NOTES ON OPTICAL INSTEUMENTS. 
211 
The concave glass of an object lens is of flint glass, and the smaller 
lenses of the eye-piece are commonly of crown glass. 
The following recipes are useful for repairs :— 
To clean hrasswork .—Take a piece of woollen rag, scrape some rotten 
stone on to it and add a little oil. Rub the brass with this, then clean 
off with a linen rag and whiting, and finish with a clean rag. 
Bead black for insides. —Take some lamp black (sold as vegetable 
black), add a few drops of gold size and a little turpentine, and mix 
well with a knife on a stone or other flat surface until it is the con¬ 
sistency of cream, and lay on cold with a camel-hair brush. 
Bead black for outsides .—Take vegetable black and shellac lacquer 
and thoroughly mix with a flat camel-hair brush (which will also do for 
laying it on). Warm the work slightly and apply the mixture; or it 
may be laid on cold and then warm sufficiently to drive off the spirit 
and melt the shellac. 
Note. —If the lacquer contains too much shellac and dries bright, 
add a little methylated spirits, but don't make the blacking too thin. 
Lacquering. —Warm the work over a spirit lamp or gas flame {not a 
Bunsen burner), then, with a flat brush, lay on shellac lacquer made by 
dissolving shellac in methylated spirit, the work must be made suffi¬ 
ciently hot to evaporate the spirit and the water it contains, but not 
so hot as to make the shellac blister, about the heat that the hand can 
bear is enough. 
Brazing and soldering. —Mix powdered borax and spelter or silver 
solder on the parts to be united and bring them to a red heat in a 
clear flame (silver solder melts at a less heat than spelter). If the 
article is small, a blow-pipe must be used. 
Soft soldering is done thus :—Wet the parts to be soldered with 
chloride of zinc or chloride of ammonia and place a small piece of 
soft solder on the joint. Heat with a blow-pipe or “ copper bit" or 
soldering iron (which latter should be first heated but not red hot, 
and coated with solder and rosin). Then rub the parts with this iron 
and supply more solder as required. 
The liniug of tea chests makes a good soft solder. 
Be-silvering sextant and range-finder mirrors .—Remove the old silver¬ 
ing with the ball of the thumb, scrape the chamfered edges of the 
glass to get rid of the old varnish. Rub the glass with a rag dipped 
in alcohol and clean with a silk handkerchief, wash leather, or linen 
rag. Then cut a piece of tin foil larger than the glass to be silvered, 
and place it on a flat stone or iron and rub it flat with a plug of cotton 
wool. Pour a little mercury on and gently rub until the whole surface 
is amalgamated. Then pour on more mercury and skim the surface till 
quite bright; then place a piece of clean paper on the edge of the 
quick-silver. Put the glass on this, and then with the finger gently 
glide the glass on to the foil. Place a weight of 2 or 3 lbs. on the 
glass and let it drain at an angle for about 12 hours (or never less 
than six hours). Out away the superfluous foil and tap the stone to 
loosen the glass. Then with a knife pare the edges, clean and cover 
the silvering with a solution of seed-lac or hard brown varnish. 
Replace the mirror, taking care not to scratch the silvering. 
