“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.