Yes; light _is_ that. Now that chair there, for example, reflects
light, just as a mirror does. It reflects vibrations. And these are
all of just a certain length, for vibrations of just that length and
moving up and down just so fast make light. The light enters the eye,
like this," tracing the rays on his sketch. "It makes a little picture
of the chair on the back of the eye, where the optic nerve is
fastened. Now the light makes the little ends of this nerve vibrate,
too--move very rapidly. And that movement is carried along the nerve
to some place in the brain--to what we call the center of sight. And
there we see the chair."

The child studied the sketch long and seriously.

"But, Padre, is the picture of the chair carried on the nerve to the
brain?"

"Oh, no, _chiquita_, only vibrations. It is as if the nerve moved just
a little distance, but very, very fast, back and forth, or up and
down."

"And no picture is carried to the brain?"

"No, there is just a vibration in the brain."

"And that vibration makes us see the chair?"

"Yes, little one."

A moment of silence. Then--

"Padre dear, I don't believe it."

"Why, _chiquita_!"

"Well, Padre, what is it that sees the chair, anyway?"

"The mind, dear."

"Is the mind up there in the brain?"

"Well--no, we can't say that it is."

"Where is it, then?"

"A--a--well, no place in particular--that is, it is right here all the
time."

"Well, then, when the mind wants to see the chair does it have to
climb up into the brain and watch that little nerve wiggle?"

The man was at a loss for an answer. Carmen suddenly crumpled the
sketch in her small hand and smiled up at him.

"Padre dear, I don't believe our outside eyes see anything. We just
think they do, don't we?"

Jose looked out through the open door. Carmen's weird heron was
stalking in immense dignity past the house.

"I think Cantar-las-horas is getting ready to sing the Vespers,
_chiquita_. And so Dona Maria probably needs you now. We will talk
more about the eye to-morrow."

By the light of his sputtering candle that night Jose sat with elbows
propped on the table, his head clasped in his hands, and a sketch of
the human eye before him. In his confident attempt to explain to
Carmen the process of cognition he had been completely baffled.
Certainly, light coming from an object enters the eye and casts a
picture upon the retina. He had often seen the photographic camera
exhibit the same