Question: What is the Hockney-Falco thesis?
About the article "Johannes Vermeer", section "Theories of mechanical aid".

Candidate passages:
The Hockney–Falco thesis is a controversial theory of art history, advanced by artist David Hockney and physicist Charles M. Falco, suggesting that advances in realism and accuracy in the history of Western art since the Renaissance were primarily the result of optical aids such as the camera obscura, camera lucida, and curved mirrors, rather than solely due the development of artistic technique and skill.
In a 2001 book, "Secret Knowledge: Rediscovering the Lost Techniques of the Old Masters", Hockney analyzed the work of the Old Masters and argued that the level of accuracy represented in their work is impossible to create by "eyeballing it".
Since then, Hockney and Falco have produced a number of publications on positive evidence of the use of optical aids, and the historical plausibility of such methods.
Part of Hockney's work involved collaboration with Charles Falco, a condensed matter physicist and an expert in optics.
While the use of optical aids would generally enhance accuracy, Falco calculated the types of distortion that would result from specific optical devices; Hockney and Falco argued that such errors could in fact be found in the work of some of the Old Masters.
Hockney's book prompted intense and sustained debate among artists, art historians, and a wide variety of other scholars.
In particular, it has spurred increased interest in the actual methods and techniques of artists among scientists and historians of science, as well as general historians and art historians.
The latter have in general reacted unfavorably, interpreting the Hockney–Falco thesis as an accusation that the Old Masters "cheated" and intentionally obscured their methods.
Physicist David G. Stork and several co-authors have argued against the Hockney–Falco thesis from a technical standpoint. 
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.
In 2000, Falco and Hockney published an analysis ("Optical Insights into Renaissance Art") of the likely use of concave mirrors in Jan van Eyck's work in "Optics & Photonics News", vol.
11.
In 2001, Hockney published an extended form of his argument in "Secret Knowledge". 
In addition to incredulity on the part of art historians and critics of modern art, some of the harshest criticism of the Hockney–Falco thesis came from another expert in optics, image processing and pattern recognition, David G. Stork.
Stork analyzed the images used by Falco and Hockney, and came to the conclusion that they do "not" demonstrate the kinds of optical distortion that curved mirrors or converging lenses would cause.
Falco has claimed that Stork's published criticisms have relied on fabricated data and misrepresentations of Hockney and Falco's theory.
Stork has rebutted this claim. 
At a scientific conference in February 2007, Falco further argued that the Arabic physicist Ibn al-Haytham's (965–1040) work on optics, in his "Book of Optics", may have influenced the use of optical aids by Renaissance artists.
Falco said that his and Hockney's examples of Renaissance art "demonstrate a continuum in the use of optics by artists from c. 1430, arguably initiated as a result of Ibn al-Haytham's influence, until today."
In the 2001 television programme and book, "Secret Knowledge", Hockney posited that the Old Masters used "camera obscura" techniques that projected the image of the subject onto the surface of the painting.
Hockney argues that this technique migrated gradually to Italy and most of Europe, and is the reason for the photographic style of painting we see in the Renaissance and later periods of art.
He published his conclusions in the 2001 book "Secret Knowledge: Rediscovering the Lost Techniques of the Old Masters," which was revised in 2006.