for polishing speculum
metal. Lord Rosso used rouge heated to a dull redness for this
purpose.

Rouge, whether made or bought, should always be washed to get rid of
grit. I ought to add that not the least remarkable fact about the
polishing is the extraordinarily small quantity of the polishing
material requisite, which suggests that the process of polishing is
not by any means the same as that of exceptionally fine grinding. Is
it possible that the chief proximate cause of the utility of rouge is
to be sought in its curious property of adhering to a rough glass
surface, causing it, so to speak, to drag the glass off in minute
quantities, and redeposit it after a certain thickness has been
attained on another part of the surface?

Sec. 63. Centering.

When a lens is ground and polished it will almost always happen that
the axis of revolution of its cylindrical edge is inclined to the axis
of revolution of its curved surfaces. Since in practice lenses have
to be adjusted by their edges, it is generally necessary to adjust the
edge to a cylinder about the axis of figure of the active surfaces.
This is best done on a lathe with a hollow mandrel.. The lens is
chucked on a chuck with a central aperture--generally by means of
pitch or Regnault's mastic, or "centering" cement for small
lenses--and a cross wire is fixed in the axis of revolution of the lathe,
and is illuminated by a lamp. This cross wire is observed by an
eye-piece (with cross wires only in the case of a convex lens, or a
telescope similarly furnished in the case of a concave lens),
also placed in the axis of rotation of the lathe.

Both cross wires are thus in the axis of revolution of the mandrel,
and the distant one (B in the figure) is viewed through the lens and
referred to the fixed cross wires at A. In general, as the lathe is
rotated by turning the mandrel the image of the illuminated cross
wires will be observed to rotate also. The lens is adjusted until the
image remains steady on rotating the mandrel and it is to give time
for this operation that a slow-setting cement is recommended. When
the image remains stationary we know that the optical centre of the
lens is in the axis of revolution, and that this axis is normal to
both lens surfaces, i.e. is the principal axis of the lens, or axis
of figure.

Fig. 54.

A much readier method, and one, in general, good enough for most
purposes, is to put a candle on the end of the lathe-bed