Secret Knowledge: rediscovering the lost techniques of the Old Masters David Hockney Thames & Hudson, 296pp, £35 ISBN 0500237859

David Hockney is blessed with an ability to write in a way that illuminates the work of others and to encourage a new intensity of seeing. In both his art and his commentaries, he has been persistently concerned with questions of perspective and image-making, vision and representation, because, as he wrote in 1993, an artist's task is not to alienate people, but to bring them closer to something: an intimate world, in Hockney's view, "seems to be more human".

Hockney's new book, Secret Knowledge, takes on a much more specific issue, but one that is no less concerned with communi- cating the deep pleasure he finds in the craft of painting. His starting point is the idea that, from as early as 1430 in Flanders, artists in the western world used optics (mirrors and lenses) as tools of their trade, and that the practice of creating projections was much more widespread than is commonly recognised in the history of art. The hunch began when he saw an exhibition of portraits by Jean-Auguste Ingres and was struck by features whose accuracy appeared at odds with their scale. Instead of passively admiring them, Hockney became absorbed in how they were done. His analysis of the visual clues - the swiftness of the lines, the detail of the head, the similarity to Warhol's traced methods - led him to believe that Ingres had used a camera lucida, a device that allows draughtsmen to plot very accurately the relative position of objects through a prism that throws their image on to the paper.

To prove his thesis, Hockney recreated more than 800 years of art by pinning photocopies of paintings on to his studio wall in California. The result was a 70-foot blast of images that brought out new similarities and differences among artists, and which allowed him to pinpoint a sudden advance in naturalism, a dramatic transformation that took place in painting at either end of the Renaissance. His conclusion was that the development of greater individuality in portraiture did not, by coincidence alone, happen at the same time as mirrors and lenses began to appear in paintings, and Secret Knowledge is the record of his quest to prove it.

The book is divided into three parts, the first being an astonishing visual argument in which paintings are compared and annotated with the infectious obsessiveness that Hockney is so good at. No scrap of evidence escapes his eye, from complex issues of distortion and perspective to obvious changes in expression and the sudden presence of left-handed people in portraits, a clue that Hockney attributes to the inversion of right-handed subjects through a lens. There follows a collection of historical material on optics and a mound of correspondence between Hockney and various experts on the subject - a fascinating, if somewhat repetitive, exchange of exuberance and caution.

Hockney, an artist who genuinely believes that painting can change the world, is not writing art off or suggesting that the acknowledged masters have in some way betrayed our faith in their genius. Only the painter's hand, he insists, makes the mark, and optics were merely an aid to what was, after all, a job. There are many flaws in his argument, as the correspondence with the art historian Martin Kemp reveals: contextual reasons for painting something a certain way are just as valid, and occasionally Hockney goes too far, elaborating discrepancies in pictures that could simply be explained by inadequacies of talent. There is also the question of how new his ideas actually are. Philip Pearlstein suggested long ago, albeit half-jokingly, that Ingres used a camera lucida, and his theory has been argued out in art journals for at least two years.

But to concentrate too much on whether Hockney is right or wrong is to miss the one great strength of the book: that it is written by a great image-maker, and that art history, in its quest for subject and context, can sometimes be disturbingly oblivious to the image itself and the techniques that made it. If Hockney's provocative thesis can have grown men turning their living rooms into Caravaggio's studio to disprove him, it is certainly enough to remind us that art is interactive, that you must work at viewing a painting. When you look at The Ambassadors through Hockney's eyes, you hear Holbein groan with the sheer bloodiness of painting the folds in a patterned tablecloth, or the words on the curved surface of a globe. And once you have done that, you'll never glance lazily at a painting again.

Nicola Upson is the author of Mythologies: the sculpture of Helaine Blumenfeld (Overlook Press, £45). She is also the crime fiction critic of the NS

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