Within the past two years, some art dealers and auction houses that handle photography have been noticing a group of rather old-looking photographs appearing from time to time on the market. Claimed to be daguerreotypes that were made by Albert Southworth and Josiah Hawes, a mid-19th-century Boston-based portrait photography team, these pictures were thought to be quite valuable until a few of them were put under a microscope. At that point, printers' dot patterns were found indicating that the pictures were simply photographed, using a 19th-century method, out of a modern-day book.

Clearly, a lot of work and knowledge of 19th-century photographic processes went into the creation of something bogus, but a growing number of people are willing to expend the energy because of the increasing amount of money being spent on antique and even contemporary photographs.

"In the past," says Steven White, a Los Angeles photography dealer, "the photography field has not been valuable enough to elicit the effort of someone to try to create fakes." However, within the past few years, certain esteemed 19th- and 20th-century pictures have sold at auction in the five- and six-digit range, raising the interest of the public and potential forgers alike in the value of photographs.

Many of the earlier photographers are unknown since they did not often sign their pictures as 20th-century photographers customarily do, so the ground has been fertile for fakes as well as incorrect attributions. Less is known about the photographers of the 19th century, who may have been hobbyists experimenting with the new medium, and the idea of a picture turning up by someone no one ever heard of is quite possible.

However, fakes of 20th-century photographs, attributed to such as Ansel Adams and Paul Strand, have also appeared regularly. In April 1986, for instance, a sale of several supposed Strand prints was consummated at Butterfield & Butterfield, the San Francisco auction house, before inquiries determined that the photographs had been misrepresented. They were in fact the work of an assistant.

Two important New York photography dealers, Peter H. Falk and Lee Witkin, had purchased a number of these pictures between 1979 and 1982, but Walter Rosenblau, a close friend of Strand and author of "A World History of Photography," was called in to examine the photographs. He found the paper, printing, vision and separate signatures to be questionable, and Falk returned the buyer's money.

Most notable photographers of the past 70 years have left some sort of documentation about which works they created, how they created them and when, and this has helped to authenticate questionable works.

Still, there can be problems.

Ansel Adams, for instance, printed hundreds of copies of his most famous 1944 image, "Moonrise, Hernandez," using different materials and making changes in the light and darkness of the picture over a 30-year period. There is no one "Moonrise, Hernandez" against which all others are to be judged, and skillful people have photographed the image to produce copies that they have sought to pass off as originals.

Photographing a photograph is the most common technique of the producers of fake works, and only the most trained eye can spot the differences.

"The key to identifying whether a work is an original or a copy is in the signs of deterioration," Grant Romer, head conservator at the George Eastman House in Rochester, N.Y., says. "Anything over 20 years old will show signs of deterioration, and many dealers will be reluctant to handle a work that looks too good for its age."

Some dealers may lack the expertise to identify a copied photograph, however, and the issue becomes even more clouded with photographers such as Brassai, Man Ray and Eugene Smith, all of whom photographed their own works when they had lost their negatives.

"I think with these photographers and with these images that no one will ever know if a work is a fake or not," says Jose Orraca, a private photography conservator. He adds that the larger problem in determining whether a photograph is original or a fake is the "lack of any hard information about which artists used which materials at what time. With painting, scientists know the kinds of pigments and canvases that certain artists used and, if the artist changed, they know how and when he changed. We don't have this in photography."

To determine a photograph's authenticity, scientists use X-ray fluorescence and electron dispersion spectroscopy -- both of which evaluate the composition of the paper as well as the chemicals involved in producing the print. This often has less applicability to 20th-century pictures.

Photographs of the 19th century were printed on types of paper that are no longer in general use, using techniques that have been replaced by more durable methods. Forgers in this realm are usually well versed in how photographers 100 years ago worked, and "their aim is as much to show that the experts aren't as expert as they like to believe as it is to make a few extra dollars," Grant Romer says.

The show-up-the-experts attitude was clearly the motive of the photographer who sold London's National Gallery a photograph of street urchins, purportedly by Lewis Carroll but actually taken by himself using children in costumes. The photographer went public with his deed, though many others have quietly kept the money.

These present-day photographers have been helped in their forgeries by "the extraordinary increase in the papers manufacturers are producing that are meant to look like 19th-century paper," says Nora Kennedy, assistant conservator at the Conservation Center for Art and Historic Artifacts in Philadelphia -- one of the few places that performs chemical analyses of photographs.

There are some others -- the Image Permanence Institute in Rochester, the Canadian Center for Architecture in Quebec and the Center for Creative Photography in Tucson, Ariz., as well as several major museums and the National Aeronautics and Space Administration -- but it is a limited field.

Fakes and forgeries may well increase with the market for photographs, and the shortage of experts and others who can authenticate a picture will undoubtedly ensure that the problems occur again and again.

Daniel Grant is a writer on the arts.