"Quite easy," replied the boy. "It is about two inches long; is made of metal, of some kind, and is used to hold matches."
"The answer needs examination. Now, tell me, first, how you judged it to be two inches long," remarked the captain.
"Why, I should easily judge that, because it wasn't as big as your hand, and not even as long as one of your fingers," said Ralph.
"In other words, you used my hand to measure it by, or, if my hand hadn't been there you would have guessed its size because you knew, approximately, the size of match boxes; is that it?" asked the captain.
"Yes," was the answer.
"Very well; how did you know it was of metal?" was the next question.
There was a broad grin on the faces of all; that was too easy; Ralph's face was all aglow as he answered: "Because I know that all match boxes are made of metal, and it looked like metal."
"You mean it was your previous knowledge; that is a fair answer," replied the captain. "But how did you know its uses?"
Ralph glanced about preparatory to making the answer, and Alfred replied: "I should say because I know that a box of that kind and of that size, and I've seen hundreds of them, is used for holding matches."
"Well answered. Now, let us sum up: the eye told you that the box was about two inches long. That is fairly accurate. You got the measurement simply by comparison. If a box had been placed within the range of your vision, so that there would be absolutely nothing with which to compare it, you could not have told by a mere observation of the eye whether it was an inch or three inches long," observed the captain.