“In order to see, we must have light.”
“What is light, Padre dear?”
“Well––light is––is vibrations. That is, rapid movement.”
“What moves?”
“A––a––a––well, nothing––that is, light is just vibrations. The pendulum of the old clock in Don Mario’s store vibrates, you know––moves back and forth.”
“And light does that?”
“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?”