Aaron Scharf's 1968 book "Art and Photography", which details evidence of the use of photographs and the camera by painters, is referred to by Hockney in his 1977 painting "My Parents" (Tate, London) in which his father is depicted attentively reading the volume. Scharf notes in his introduction that in 1568 Daniele Barbaro, the Venetian writer on architecture, recommended the camera obscura as an aid to artists: "By holding the paper steady you can trace the whole perspective outline with a pen, shade it, and delicately colour it from nature." As described in "Secret Knowledge", in January 1999 during a visit to the National Gallery, London, Hockney conceived of the idea that optical aids were the key factor in the development of artistic realism. He was struck by the accuracy of portraits by Jean Auguste Dominique Ingres, and became convinced that Ingres had used a camera lucida or similar device. From there, Hockney began looking for signs of the use of optical aids in earlier paintings, creating what he called the "Great Wall" in his studio by organizing images of great realistic art by time period. What he saw as a sudden rise of realism around 1420, combined with Charles Falco's suggestion that concave mirrors could have been used in that period to project images, was the germ of the Hockney–Falco thesis.