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Recently it has been claimed that some early Renaissance painters used concave mirrors to project real inverted images onto their supports (paper, canvas, oak panel, ...) which they then traced or painted over, and that this was an important source of the increase in realism in European painting around 1420. Key exhibits adduced as evidence in support of this bold theory are a pair of portraits by Jan van Eyck of Cardinal Niccolo Albergati(&?) - a silverpoint study of 1431 and a larger oil of 1432. The contours in these two works bear striking resemblance in form (after being appropriately scaled) and at least one distinctive "relative shift" - evidence that has led proponents of the projection theory to claim that the oil was copied by means of an epidiascope or primitive opaque projector, the shift due to an accidental "bump" during the copying process.
We find several difficulties with this optical explanation: there are at least two relative shifts (one horizontal and one vertical), the latter being somewhat unlikely given the putative projection equipment and setup; these shifts are in the ratio of distances of nearly 1:2, a ratio that has no natural role in the projection explanation; any accidental "bump" would surely have been noticed by van Eyck, and if so desired, corrected by him; recent analysis shows physical evidence (tiny pinpricks presumably from mechanical compass) consistent with mechanical transfer that has no role in the optical explanation; and several other points. The fidelity of the copy as well as the direction and relative magnitudes of these shifts are, however, consistent with the use of a familiar grid construction and with mechanical transfer using drawing compass and ruler or Reductionszirkel. Further, there are prominent vertical Bruchkanten (fold or fraction) lines on the grounded paper in the silverpoint study whose orientation and separation have no natural role in an optical theory, but have a plausible role in other explanations. Our rebuttal to the projection theory for these works is supposed by considertaion of hte lack of documentary evidence from both artists and scientists, of surviving optical devices, and of the artistic goals and established painting praxis in the early Renaissance.
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