"Now then, we'll begin," said Uncle Richard; "and the first thing is to
make our mould or gauge, for everything we do must be so exact that we
can set distortion at defiance. We must have no aberration, as
opticians call it."
"Begin to polish the glass, uncle?"
"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.
A couple of strong nails at the sides of the board were sufficient, and
then he led the way in.
"Now, Tom, take that ball of twine and the hammer, and go up to the top
window, open it, and look out."
The boy did not stop to say "What for?" but ran up-stairs, opened the
window, and looked out, to find his uncle beneath with the long rod.
"Lower down the end of the string," he cried; and this was done, Tom
watching, and seeing it tied to the end of the rod where the brass nail
stuck through.
"Haul up, Tom."
The twine was tightened, and the end of the rod drawn up till Tom could
take it in his hand.
"Now take away the string."
This was done.
"Get your hammer."
"It's here on the window-sill, uncle."
"That's right. Now look here: I want you to lean out, and drive that
nail in between two of the bricks, so that this marking-point at my end
may hang just a few inches above the bottom of my piece of zinc. I'll
guide it. That's just right. Now drive in the nail."
"Must come an inch higher, so that the nail may be opposite a joint."
"Take it an inch higher, and drive it in."
This was done, and the rod swung like an immensely long wooden pendulum.
"That's right," cried Uncle Richard; "the nail and this point are
exactly twenty-four feet apart. Now keep your finger on the head of the
nail to steady it while I mark the zinc."
Tom obeyed, and looked down the while, to see his uncle move the rod to
and fro, till he had scored in the sheet of zinc a curve as neatly and
more truly than if it had been done with a pair of compasses.
"That's all, Tom," he said. "Take out the nail and lower the rod down
again carefully, or it will break."
All this was done, and Tom descended to find that both the rod and the
sheet of zinc had been carried in, the latter laid on the bench, and
displaying a curve deeply scratched upon it where the sharp-pointed
bradawl had been drawn.
"There, Tom," said Uncle Richard, "that curve is exactly the one we have
to make in our speculum, so that we may have a telescope of twelve feet
focus. Do you understand?"
"No," said Tom bluntly.
"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?"
"So I do, Tom."
"But that will only keep the pieces flat."
"I beg your pardon, my boy. If we rub and grind them as I propose, one
of the discs will be rounded and the other hollowed exactly as I wish."
Tom stared, for this was to his way of thinking impossible.
"Are you sure you are right, uncle? Because if you are not, it would be
so much trouble for nothing."
"Let's prove it," said Uncle Richard, smiling. "Go to the kitchen door,
and ask the cook for a couple of good-sized pieces of salt and the
meat-saw."
The cook stared, but furnished the required pieces, which were soon
shaped into flat slabs with the saw. Then a sheet of newspaper was
spread, and one of the flat pieces of salt placed upon the other.
"There you are, Tom," said his uncle. "I want you to see for yourself;
then you will work better. Now then, grind away, keeping the bottom
piece firm, and the top going in circular strokes, the top passing half
off the bottom every time."
Tom began, and worked away, while from time to time the lower piece was
turned round.
"Nice fine salt," said Uncle Richard; "cook ought to be much obliged."
"It will be as flat as flat," said Tom to himself, "but I don't like to
tell him so."
"There, that will do," said Uncle Richard, at the end of ten minutes.
"Now then, are the pieces both flat?"
"No, uncle; the bottom piece is rounded and the top hollowed, but I
can't see why."
"Then I'll tell you: because the centre gets rubbed more than the sides,
Tom. There, take paper and salt back, and we'll begin."
Tom caught up the paper, and soon returned, eager to commence; and after
a little instruction as to how he was to place his hands upon the top
glass, Uncle Richard placed himself exactly opposite to his nephew, with
the upturned cask between them.
"Now, Tom, it will be a very long and tedious task with this great
speculum; hot work for us too, so we must do a bit now and a bit then,
so as not to weary ourselves out. Ready?"
"Yes, uncle."
"Then off."
"It will be a tiresome job," thought Tom, as, trying hard to get into
regular swing with his uncle, the top glass was pushed to and fro from
one to the other; but at each thrust Uncle Richard made a half step to
his left, Tom, according to instructions, the same, so that the glass
might be ground regularly all over. At the end of a quarter of an hour
it was slid on one side, and more water and sand applied. Then on
again, and the grinding continued, the weight of the glass making the
task very difficult. But Tom worked manfully, encouraged by his uncle's
assurance that every day he would grow more accustomed to the work, and
after two more stoppages there was a cessation.
"There!" cried Uncle Richard; "one hour's enough for the first day. It
wants faith to go on with such a business, Tom."
As he spoke the future speculum was carefully lifted off the lower one,
sponged with clean water, and on examination proved to be pretty well
scratched in the middle in a round patch, but the marks grew less and
less, till at the edge of the glass it was hardly scratched at all.
"There, you see where we bite hardest," said Uncle Richard; "now we'll
give it a rest, and ourselves too."
"But we shall never get done like this," cried Tom.
"Oh yes, we shall, boy; and I'm not going to leave off our work. Let's
see: this we must call the workshop, the floor above our laboratory, and
the top of course the observatory. Now then, let's go up into our
laboratory, and I'll give you a lesson in elutriation."