t together to the mill-yard, where
the footprints were pointed out.

"Have to keep the door carefully locked, Tom," said Uncle Richard.
"Hah! capital! the sun will be shining right through that window in a
few minutes."

They entered the workshop, where a bench was drawn opposite to the last
window, and about twelve feet away. To this, with Tom's help, the
partly-polished speculum was borne.

"Not very bright for a reflector, Tom," said Uncle Richard. "What am I
to do to make it brighter?"

"Go on polishing, uncle."

"Ah, but I want to test it this morning, to see if we have a good
curve," said Tom's elder, smiling. "Fill the sponge with clean water
and bring it here."

This was done, and the finely-ground surface was freely wetted, with the
effect that it became far more luminous directly.

"Now, Tom," said his uncle, "I'm going to show you something in
reflection. The sun is not quite high enough for the speculum, so give
me that piece of looking-glass."

This was handed to him, and he held it on high, so that the low-down sun
shone into it, and a reflection was cast from it back upon the wall just
above the window.

"See that?"

"Yes, uncle. Done that many a time. Used to call it making
jack-o'-lanterns."

"Well, that is the effect of a reflection from a flat or plane surface;
the rays of light strike back at the same angle as they hit the surface.
Now then, I'll show you what happens from a curved surface."

He passed the sponge rapidly over the ground speculum again, so as to
glaze it--so to speak--with water, raised it upon its edge with the
carefully-ground face directed at the window just as the sun rose high
enough to shine in; and then by turning the great mirror slightly, the
light reflected from it struck upon the wall at the side of the window.

"Now, Tom, what do you see?"

"A round spot of light about as big as a two-shilling piece," said the
boy.

"Yes; all the rays of light which fall upon our mirror, gradually drawn
together to where they form an image of the sun. It is only dull, my
boy, but so far finely perfect, and we can say that we have gone on very
successfully."

As he spoke he laid the mirror down upon its back.

"Is that all you are going to do?" asked Tom.

"Yes; I can test it no better till it is more advanced, my boy. It may
seem a little thing to you, but it is enough to show me that we may go
on, and not begin our work all over again. Now for a good turn