first, the results are so good that it well
repays the few hours' work it requires to master the few difficulties
it presents."

Fig. 58.

Sec. 70. Casting Specula for Mirrors.

According to Sir H. Grubb (loc. cit.) the best alloy is made of four
atoms of copper and one of tin; this gives by weight,
copper 252, tin 117.8.

The copper is melted first in a plumbago crucible; the tin is added
gradually. Of course, in the process of melting, even though a little
fine charcoal be sprinkled over the copper, some loss of that metal
will occur from oxidation. It is convenient in practice, therefore to
reserve a portion of the tin and test the contents of the crucible by
lifting a little of the alloy out and examining it.

The following indications may be noted: When the copper is in excess
the tint of the alloy is slightly red, and the structure, as shown at
a fractured surface, is coarsely crystalline. As the proper
proportions are more nearly attained, the crystalline structure
becomes finer, the colour whiter, and the crystals brighter. The
alloy is ready for use when the maximum brightness is attained and the
grain is fine.

If too much tin be added, the lustre diminishes. The correct
proportion is, therefore, attained when a further small addition of
tin produces no apparent increase of brightness or fineness of grain.
About three-quarters of the tin may be added at first, and the other
quarter added with testing as described. The alloy is allowed to cool
until on skimming the surface the metal appears bright and remains so
without losing its lustre by oxidation for a sensible time; it will
still be quite red-hot.

Fig. 59. Fig. 60.

As the speculum alloy is too difficult to work with ordinary tools, it
is best to cast the speculum of exactly the required shape and size.
This is done by means of a ring of iron turned inside (and out) and on
one edge. This ring is laid on a plate of figured iron, and before
the metal is poured the plate (G) (Figs 59 and 61) is heated to, say,
300 deg. C. In order to avoid the presence of oxide as far as possible,
the following arrangements for pouring are made. A portion of the
lower surface of the ring is removed by radial filing until a notch
equal to, say, one-twentieth of the whole circumference is produced.
This is cut to an axial depth of, say, half an inch.

A bar of iron is then dovetailed loosely into the not