* [http://search.eb.com/eb/article?tocId=46639 Space perception]. ''Encyclopædia Britannica'' from Encyclopædia Britannica Online. Accessed June 12, 2005.
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* [https://search.eb.com/eb/article?tocId=46639 Space perception]. ''Encyclopædia Britannica'' from Encyclopædia Britannica Online. Accessed June 12, 2005.