until
breakfast-time. Two good hours' work ought to produce some effect."

The lower disc, now become convex, was wetted and lightly touched over
with number five emery, which seemed soft enough for anything; the
well-advanced mirror was turned over upon it, fitting now very closely,
and with the sweet morning air floating in from the pine-woods, and the
birds singing all around, the monotonous task went on with its
intermissions till Uncle Richard gave the final wash off, and
said--"Breakfast!"

They were so far advanced now that Tom was as eager to recommence as his
uncle, and by that evening so much progress had been made that the
setting sun was made to shine in upon it, to be reflected back in a
bright spot on the wall without the aid of water; while two evenings
later, when the great round glass was stood all dry the polish upon it
was limpid, and seemed to be as pure as could be. There was not the
faintest scratch visible, and Tom cried in triumph--

"There, now it is done! Oh, uncle, it is grand!"

"Grand enough so far, my boy. We have succeeded almost beyond my
expectations; but that is only the first stage."

"First--stage?" faltered Tom, looking at his uncle aghast.

"Yes, boy; we have succeeded in making a beautiful spherical concave
mirror, which could be of no use whatever for my purpose."

"Then why did we make it?" cried Tom. "For practice?"

"No, boy; because it is the step towards making an ellipse, or, as they
call it when shaped for a reflecting telescope, a parabola. You know
what an ellipse is?"

"Gooseberry," said Tom bluntly.

"Gooseberry-shaped," said his uncle. "Well then, what is a parabola?"

"One of those things we used to learn about in geometry."

"Good. Well, to-morrow we must begin polishing, or rather I must, to
turn our glass from a spherical-curved mirror into a parabola."

"You'll let me help, uncle?"

"As much as I can, my boy; but the amount I have to polish off, in what
is called figuring, is so small that it requires the most delicate of
treatment, and first of all we have to prepare a small polisher to work
by hand."

This was formed of lead in the course of the next day--a nearly flat but
slightly convex disc, with a handle upon its back, and when made
perfectly smooth it was covered with hot pitch, which, as it cooled, was
made to take the exact curve of the nearly finished mirror, by being
pressed upon it, the pitch yielding sufficiently for the pur