Sir

David G. Stork, in his review of the book Optics, Instruments and Painting, 1420–1720: Reflections on the Hockney–Falco Thesis (“Tracing the history of art” Nature 438, 916–917; 200510.1038/438916b) reports the claim that appropriate concave mirrors to project optical images onto a canvas for tracing could not have existed in the fifteenth century. He concludes that the book may “close the door on the Hockney–Falco tracing thesis”.

But our thesis is about the use of optics, not necessarily concave mirrors (although there is strong circumstantial evidence for mirrors). David Hockney and I have explicitly written during the past five years that none of the optical evidence we have found allows us to distinguish between the use of refractive versus reflective optics; either or both types are possibilities.

The spectacles and the magnifying glass in Tommaso da Modena's 1351 frescos would each have been able to project appropriate images, as would the spectacles in van Eyck's van der Paele altarpiece of about 1435.