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On May 24,1832, the Swiss naturalist L. A. Necker (1832, p. 336) wrote a letter to Sir David Brewster in which he described the "sudden and involuntary change in the apparent position of a crystal" during its observation. Illustrating his point (see Figure 1), Necker stated that repeated observations of the same figure resulted in a change in its configuration. At one moment the face ACDB was foremost with the face XDC behind it, and at another moment XDC came forward while ACDB receded. Admittedly puzzled, and confessing that for a long time he had been "at a loss to understand the reason of the apparently accidental and involuntary change," Necker added that the only thing which he could observe was that "at the time the change took place, a particular sensation was felt in the eye." This observation was, however, an important one, because it proved to him that an optical and not merely mental operation was involved. It was "an involuntary change in the adjustment of the eye for obtaining distinct vision (pp. 336, 337)," which explained the phenomenon. It was the moving eye searching for a clear vision which accounted for the observation, and, in a test of this explanation, Necker discovered that he could bring about the change by focusing his vision in a certain fashion.

DOI: 10.1007/978-1-4613-3802-4_1

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