Did the Dutch masters cheat?

If this fascinating book does nothing else, it sharpens the way you look at paintings.

But it does more than that. Its author, David Hockney, proposes a radical reappraisal of one crucial aspect of the history of Western painting: how many old masters used "optical tools."

Throughout this exceptionally well-designed book, Hockney argues that much Western painting was essentially "optical." Its naturalism is based on light, not on some sudden improvement in painterly skill. He means that many artists - and he is convinced that the process began two centuries earlier than conventional art history would allow - resorted to optical devices whenever available.

This need not diminish our admiration for their achievements. But it does redirect the way we might look at paintings.

A scientist, knowing of Hockney's interest in how old masters used optical instruments, one day casually let fall an apple. He pointed out that a concave mirror can project an image just as a lens does. This meant artists at least two centuries earlier than Vermeer could have used optics.

As a painter himself, Hockney had always been particularly puzzled by a chandelier in Jan van Eyck's renowned picture the "Arnolfini Wedding" (1434). He could never grasp how van Eyck painted this complex object - particularly as X-rays show there is no underdrawing. But suppose he could have worked from a projected image? At that date, a lens would not have been available. But a mirror would.

This discovery led Hockney to the conclusion that more than a few old masters whose paintings have "an optical look" may well have used mirrors or lenses or a combination. His astonishing list includes Hugo van der Goes, Antonello de Messina, Dürer, Raphael, Moroni, Bronzino, Holbein, and - among later artists - Caravaggio, Hals, Velázquez, Van Dyck, and Chardin. He demonstrates his argument mainly by looking hard at the character of their works. He excels at this, and the book is full of gorgeous prints and helpful illustrations.

Hockney is not just reiterating established facts - although he revealingly reexamines some pertinent but known facts - for example, the obsession of Vermeer in the 17th century or Canaletto in the 18th with the "natural" and "real" effects produced by the camera obscura. This device used lens and mirror to project a scene or subject onto a flat surface in a dark room or a small box.

Hockney's practical demonstrations of its use are a revelation. He also discusses the use by 19th-century artists of the much more portable device, the camera lucida. But he goes further than discussion. He learned to use it himself. And he shows convincingly how it was used by even such a distinguished artist as Ingres

(1780-1867).

Photography followed these earlier lens devices, giving artists yet another optical tool, though eventually they veered away from the lens in adverse reaction to the monopoly photography claimed on "realism."

Hockney's contention is that lens-based or lens-induced images (which developed into film and TV as well as the increasingly banal ubiquity of still photography itself) may once have inspired artists like Vermeer to make astonishing, magical paintings using hand and thought as well as eye. But that today "the lens" has become so dominant that it has virtually stopped us from seeing freshly at all. It has rendered imagemaking a dull business, and painting almost impossible.

There is exaggeration in this. Hockney himself confesses that he remains in love with images and painting. And as far as his "discovery" in this book is concerned, it's clear he can hardly contain his excitement. He makes much of the fact that he made this discovery because he is "a practitioner" - an artist - and not an art historian.

Hockney feels artists use their eyes in ways that art historians rarely do. He accuses the latter group of often ignoring how paintings were done. Their concern is written documentary evidence - and, one may add, social history. They forget something that Roberto Longhi, one old-school art historian, did not: "Paintings are primary documents."

Whether or not his explanation for a lack of direct written evidence that specific artists used optical tools will convince art historians remains to be seen. Secrecy about techniques, he argues, is understandably endemic to artists even today. In the time of guilds and the Inquisition, artists would have been positively clandestine.

Exceptions prove the rule, of course. David Hockney, as his enthralling book more than shows, has always been admirably unsecretive.

Christopher Andreae writes for the Monitor from Glasgow, Scotland.