“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 purpose.
This done the pitch was scored across and across, till it was divided into squares, with little channels between them, so that the polishing powder and water might run freely between; then a final pressure was given upon the mirror and the implement was left to harden till the next day.