in the box and
is well pressed down so as to half fill it. The other half must be
conveniently ready to hand. As soon as possible after casting, the
speculum is thrown into the box, covered over with the sawdust, and
the lid is put on.

The object in having the box nearly air-tight is to avoid
air-currents, which would increase the rate of cooling. A speculum a
foot in diameter may conveniently take about three days to anneal, and
should be sensibly warm when the box is opened on the fourth day. For
larger sizes longer times will be required. We will say that the
sawdust thickness on each side must be proportional to the dimensions
of the speculum, or may even increase faster with advantage if time is
of no moment.

The process of annealing may be considered successful if the disc does
not fly to pieces in working; it is to be worked on the chilled side.
The object of giving the chill the approximate counterpart form will
now appear; it saves some rough grinding, and causes the finished
surface to be more homogeneous than it would be if the centre were
sunk by grinding through the chilled surface.

In 1889 I learned from Mr. Schneider, Professor Row-land's assistant
at Baltimore, that in casting specula for concave gratings a good deal
of trouble had been saved by carrying out the operation in an
atmosphere consisting mostly of coal gas. It was claimed that in this
way the presence of specks of oxide was avoided. I did not see the
process in operation, but the results attained are known and admired
by all experimenters.

Sec. 71. Grinding and polishing Specula.

The rough grinding is accomplished by means of a lead tool and coarse
emery; the size of grain may be such as will pass a sieve of 60
threads to the inch. The process of grinding is quite similar to that
previously described, but it goes on comparatively quickly. The rough
grinding is checked by the spherometer, and is interrupted when that
instrument gives accordant and correct measurements all over the
surface.

The fine grinding may be proceeded with by means of a glass-faced tool
as before described, or the labour may be reduced in the following
manner. A slate tool, which must be free from green spots (a source
of uneven hardness), is prepared, and this is brought nearly to the
curvature of the roughly ground speculum, by turning or otherwise. It
is finished on the speculum itself with a little flour of emery. The
fine grinding is th