"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. "Do you mean I wouldn't have been able to tell the size of a match safe?" asked Alfred. "No; I didn't say match safe. I said if _a box_ had been exhibited before you. If I had asked you the size of a match safe it wouldn't have been necessary for me to exhibit it; your knowledge of the general sizes of match safes would have enabled you to answer me without even glancing at it. Isn't that true?" asked the captain. "I see what you mean now," said Ralph. "It is previous knowledge that aids the eye." "That is the idea," said the captain. "Now, proceed with the next question. Why did you say it was made of metal?" "Because I never saw a match safe that wasn't made of some kind of metal," said Ralph. "Did the eye tell the truth?" said the captain, taking out his knife and opening it. "You will see it is made of _papier mache_, merely colored to look like metal. The eye was a gay deceiver; don't you think so?" This was too much for the men; there was a sally of laughter in which the boys joined with the greatest glee. "But we are not through with this investigation. We have been talking about a match safe. The sergeant here is a very wise person, and has had a fine education, so I am going to ask him whether it looks like a match safe," said the captain. "I should say so; that is about the only thing that occurs to me," he answered. "Are you relying on your eyes, or what?" asked the captain. "Well,—on my eyes and on my previous knowledge," answered the sergeant. "Then you are doubly wrong," said the captain, as he opened the lid, and exposed the interior of the case filled with tablets. "It is not a match safe; was never intended for one, and was never used for that purpose. Have I proven my case?" The company applauded the clever manner in which the captain explained the subject.