“Never mind—you soon will. It means that when we have ground out the glass so that it is a hollow of that shape, all the light reflected will meet at a point just twelve feet distant from its surface. Now we have begun in real earnest.”
He now took a keen-edged chisel, and pressing the corner down proceeded to deepen the mark scored in the zinc with the greatest care, until he had cut right through, forming the metal into two moulds, one of which was to gauge the lower disc, the other the upper. The edges of these were then rubbed carefully together as they lay flat upon the bench, till their edges were quite smooth; then some of the unnecessary zinc was cut away, a couple of big holes punched in them, and they were hung upon a couple of nails over the bench ready for use.
“Next thing,” cried Uncle Richard, “is to begin upon the speculum itself, so now for our apparatus. Here we have it all: a bowl of fine sifted silver sand, a bucket of water, and a sponge. Very simple things for bringing the moon so near, eh?”
“But is that all we want, uncle?”
“At present, my boy,” said Uncle Richard, proceeding to wet some of the sand and pretty well cover the disc of glass fixed upon the cask-head. “That’s for grinding, as you see.”
“Yes, uncle; but what are you going to rub it with?”
“The other disc. Here, catch hold. Be careful.”
Tom obeyed, and the smooth piece of plate-glass was laid flat upon the first piece, crushing down the wet sand, and fitting well into its place.
“Now, my boy, if we rub those two together, what will be the effect?”
“Grind the glass,” said Tom. “I once made a transparent slate like that, by rubbing a piece of glass on a stone with some sand and water. But I thought you wanted to hollow out the glass?”