The famous duck-rabbit ambiguous image. Is it a duck? Is it a rabbit?

"The subject of a gestalt demonstration knows that his perception has shifted because he can make it shift back and forth repeatedly while he holds the same book or piece of paper in his hands. Aware that nothing in his environment has changed, he directs his attention increasingly not to the figure (duck or rabbit) but to the lines of the paper he is looking at. Ultimately he may even learn to see those lines without seeing either of the figures, and he may then say (what he could not legitimately have said earlier) that it is these lines that he really sees but that he sees them alternately as a duck and as a rabbit. ...As in all similar psychological experiments, the effectiveness of the demonstration depends upon its being analyzable in this way. Unless there were an external standard with respect to which a switch of vision could be demonstrated, no conclusion about alternate perceptual possibilities could be drawn." -- Thomas Kuhn, The Structure of Scientific Revolutions (3rd edn., p. 114).

• Source: Jastrow, J. (1899). The mind's eye. Popular Science Monthly, 54, 299-312.

	这个媒体文件在美国属于公有领域。这适用于版权过期的美国作品，通常因为其首次出版于1929年1月1日之前，或即便当天或以后出版，其并未包含版权通告或获得版权回溯。查看这个页面可以获取到更多的解释。
	这个图片在美国以外可能尚未处于公有领域；尤其在对美国作品不适用较短期限法则的国家和地区，如加拿大、中国大陆（不含香港及澳门）、德国、墨西哥和瑞士。创作者和发表年份是关键信息，因此必须提供。参见维基百科:公有领域及维基百科:版权信息以获取更多细节。