“Not yet. Fetch those two pieces of lath.” Tom fetched a couple of thin pieces of wood, each a little over twelve feet long. These were laid upon the bench and screwed together, so as to make one rod just over twenty-four feet long.
Then at one end a hole was made, into which a large brass-headed nail was thrust, while through the other end a sharp-pointed bradawl was bored, so as to leave its sharp point sticking out a quarter of an inch on the other side.
“So far so good,” said Uncle Richard. “Do you know what we are going to do, Tom?” Tom shook his head.
“Strike the curve on that piece of zinc that we are to make our speculum.”
“Curve?” said Tom; “why, it’s quite round now.”
“Yes; the edge is, but we are going to work at the face.”
“But arn’t you going to polish it into a looking-glass?”
“Yes; but not a flat one—a plane. That would be of no use to us, Tom; we must have a parabolic curve.”
“Oh,” said Tom, who only knew parabolas from a cursory acquaintance with them through an old Greek friend called Euclid.
“Be patient, and you’ll soon understand,” continued Uncle Richard, who proceeded to secure the sheet of zinc to a piece of board by means of four tacks at its corners, and ended by carrying it out, and fixing the board just at the bottom of the border, close to the window.