"But, Jim, that's impossible!"
"Why is it impossible?"
"It would imply the invisibility of a tangible substance; of a solid."
"What of it?"
"Why, there isn't any such substance. Nothing of the sort exists."
Carpenter pointed to one of the windows of his laboratory.
"Does that window frame contain glass or not?" he asked.
I strained my eyes. Certainly nothing was visible.
"Yes," I said at a venture.
He rose and thrust his hand through the space where the glass should have been.