While this conversation was going on, Ronald and Otis had been deeply engaged with the toy sent by Oscar’s brother. It consisted of a circular card, on one side of which was painted a bird-cage, and on the other a bird. There were strings on each side of the card, by which it could be rapidly twirled round, which operation made the bird look as if he were actually in the cage. The engraving which we give of this little toy necessarily represents it as composed of two cards, but there is only one. Do you know why the bird is represented upside down? Did you ever notice that the top of one side of a coin is always the bottom of the other side? Both of these facts are to be explained on the same principle. We do not turn over a coin as we do the leaf of a book, but we reverse the top and bottom. As the card revolves, the bird will of course show himself right side up.
“Ronald, can you explain why it is that the bird looks as if he were in the cage?” asked Marcus, after he had examined the toy.
“I suppose it’s because the card revolves so fast that we see both sides at once,” replied Ronald.
“That is hardly a philosophical explanation,” said Marcus. “The true reason is, the image of the bird is brought to the retina of the eye before the image of the cage has passed away, and so both unite, and produce the image of a bird and cage. The image of an object on the retina does not vanish the instant the object is withdrawn, but is retained a brief period afterward. This is the reason that two objects may be seen in the same place at once, while each of them is presented to the retina but half the time.”
Aunt Fanny said she had seen a mouse and a trap represented in this way. She also suggested that the body and legs of a man might be painted on one side, and his arms and head on the other; or a horse on one side and his rider on the other; or a portrait, and a frame; or a cell, and a prisoner; and several other devices were named.
It was settled that Oscar should avail himself of his father’s invitation, and spend his vacation in Boston. He promised Jessie that he would try to find Sam, and persuade him to return to Vermont. He also promised Ronald that he would take charge of sundry cakes of maple sugar which the latter desired to send to Ralph, in return for his present.
This invitation home was as unexpected as it was agreeable to Oscar. He had not anticipated visiting Boston until the next autumn. It was judged, however, that he had become so fixed in his good purposes and habits, there would be no risk in allowing him to return for a week or two to the scene of his former temptations and misdeeds.
CHAPTER XVIII.
THE SURPRISE PARTY.
The twentieth of May at length came, and the academic term closed with a searching examination of the several classes. It went a little deeper than faces or dresses, and revealed to the assembled magnates of the town something of the daily habits, the intellectual standing and the private character of each pupil. The result, as a whole, was quite favorable to the institution, and there were very few of the scholars who positively reflected no credit upon it. It was evident enough where the blame lay, in these cases—the school register told the story.