"Yes," answered Ralph. "I think it's pretty near the truth. Sometimes even the upturned face of a man will attract attention, although the face makes a mighty small speck, but I believe that fellows who think they can see a man's face at a distance of 5,000 feet, either use a field glass, or the man below happened to be wearing eye glasses, for they make fine reflectors for the airmen." "But those who are flying get birds' eyes, after a time," said Alfred. "What is that?" asked the captain, who appeared at the door. "Do you think a man's eyesight grows keener by flying, after he is at it for a time?" "It seems so to me," answered Ralph. The captain shook his head. "I think that is a misapprehension. The eyesight does not become sharper or more acute." "Then how is it that I can now see things that I could not notice when we first began to fly?" asked Alfred. "Observation! observation, my boy! You can't see one whit better today than you could the first time you went aloft," said the captain. "The eye is a very deceptive thing,—you laugh at the statement,—well, I'm going to prove it. In everything you see the judgment is not formed by what the eye tells you, but by your knowledge, your habit of observation and application growing out of previous experiences." "Pardon me, Captain. Do you mean to say that the eye doesn't correctly tell you distance or size or what the object really is?" asked Alfred. "That's exactly what I mean," replied the captain. "Well, that's a new idea to me," said Ralph. "Suppose we examine that. I have an article here,—a box, in my hand. Tell me, Ralph, how large it is, what it is made of, and what it is used for?" said the captain. "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."