I first wrote about the controversial thesis of this book back in January of 2000, when my "Pick" was an article about David Hockney by Lawrence Wechler in the New Yorker. With the publication of this very attractive, large-format book, you can look for yourself at the evidence that he argues shows that many of the great master painters secretly used optical devices to help produce their work. The thing that I most like about "Secret Knowledge" is that the first half presents visual examples with minimal text. You can follow the thesis and consider the strengths and weaknesses of the argument, examining each of the magnificent plates for yourself. In the second half, Hockney provides historical background for the camera obscura and the camera lucida, and his correspondence with art historians, museum curators, and scientists around the world. This is a relatively expensive book that is worth the price, on both aesthetic and intellectual grounds. A less expensive book that examines the use of optical technology by Vermeer is "Vermeer's Camera : Uncovering the Truth Behind the Masterpieces" by Phillip Steadman, published by Oxford University Press.