Telescopic Mirrors and Object Lenses. 87 
and the other concave, and adapted to the intended curvature of 
the mirror. With these gages are formed a wooden model, if re- 
quired, for casting the mirror; and also a pair of spherical tools, 
made of pewter, consisting of five parts of lead and one of tin; 
the convex tool having the larger surface. The proportion of 
width usually adopted is that of ten to eight and a half; but this 
depends partly on the length of the stroke which the mirror is 
intended to perform; and on some other data which cannot be 
estimated in theory. The concave leaden tool is similar to the 
mirror, and may be cast in the same mould. But for grinding 
lenses, it is the opinion of the workmen that this tool should 
be made of brass. The composition of speculum metal is of copper 
and tin, mixed in the proportion of fourteen and a half ounces of 
grain tin to two pounds of copper, with a small addition of arsenic. 
This produces a very brilliant metal, but is said to be sometimes 
porous. This inconvenience is remedied by first pouring the metal 
into an ingot. Other proportions of tin are given by different 
authors ; to which are sometimes added brass and silver in small 
quantities. The copper must be first melted by itself, and the 
tin poured upon it in a state of fusion ; a flux may then be added, 
and the whole well stirred with a wooden stick. The last portions 
of tin are added gradually; and between each addition a few drops 
of the metal are cooled, to observe the brilliancy by fracture, which 
is called tasting the metal. 
The extreme brittleness of this composition makes the casting 
very difficult and hazardous. Of several mirrors cast successively, 
with every attention to the printed directions, scarcely any were 
found perfect enough to repay the trouble of grinding. The dif- 
ficulty of casting and annealing may however be greatly lessened 
by pouring the meta], carefully skimmed, into a mould of cast iron, 
heated below redness. The mould may be heated over a small 
stove, and continue, in the same position, gradually cooling for 
