Book 6 of the De aspectibus takes on a dual significance as an end to Alhacen’s reflection-analysis, not simply because it concludes that analysis but because it represents the ultimate goal for it. Accordingly, Alhacen’s purpose is to apply the cathetus-rule to an analysis of the various misperceptions that arise in the seven types of mirrors chosen for study in the previous books. Some of these misperceptions are common to all mirrors, an example being image-displacement.  Some, however, are specific to the shape of the mirror, concave mirrors being a major source because of magnification, image-inversion, and image-location.

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