en carried on by means of slate dust and water, the
slate tool being the grinder. The tool is, of course, scored into
squares on the surface.

If the casting process has been carried out successfully, the rough
grinding may take, say six hours, and the fine grinding say thirty
hours for a disc a foot in diameter. The greatest source of trouble
is want of homogeneity in the casting, as evidenced by blowholes, etc.
In general, the shortest way is to discard the disc and start afresh
if there is any serious want of perfection in the continuity or
homogeneity of the metal.

Fig. 62.

The finely ground surface must, of course, be apparently correct in so
far as a spherometer (with 3 inches between the legs for a disc 1 foot
in diameter) will show. Polishing and figuring are carried out
simultaneously. Half an hour's polishing with a slate-backed pitch
tool and rouge and water will enable an optical test to be made. The
most convenient test is that of Foucault, a simple appliance for the
purpose being shown in the figure (62). It essentially consists of a
small lamp surrounded by an opaque chimney (A) through which a minute
aperture (pin-hole) is made. A small lens may be used, of very great
curvature, or even a transparent marble to throw an image of the flame
on the pin-hole.

A screen (B) is placed close to the source, and is provided with a
rocking or tilting motion (C) in its own plane. The source and screen
are partly independent, and each is provided with a fine adjustment
which serves to place it in position near the centre of curvature.
The screen is so close to the pin-hole in fact that both the source
and a point on the edge of the screen may be said to be at the centre
of curvature of the mirror. The mirror is temporarily mounted so as
to have its axis horizontal, in a cellar or other place of uniform
temperature.

The final focussing to the centre of curvature is made by the fine
adjustment screws; the image may be received on a bit of paper placed
on the screen and overlapping the edge nearest the source. The screws
are worked till the image has its smallest dimension and is bisected
by the edge of the screen. The test consists in observing the
appearance of the mirror surface while the screen is tilted to cut off
the light, as seen by an eye placed at the edge of the screen, a
peephole or eye lens being provided to facilitate placing the eye in a
correct position. The screen screws are work