Ninger purchased high-quality bond paper from Crane & Company, Dalton, Mass., the company that still makes the paper for the United States Treasury. Then he would cut the paper to the same size as the notes he was copying and place the cut paper in a weak coffee solution. After soaking the paper and placing it dry on top of the real note he was about to counterfeit, he placed both on a sheet of glass.

With both pieces stuck together, Ninger could easily trace the details of the underlying bill. After the bill had been traced, Ninger went over the now-dry bond paper with a camel's hair brush, coloring the bills and even duplicating the look of the blue and red threads embedded in the paper.

Ninger went a step further than most. He would not try to duplicate the intricate details, but would only give the beholder an impression of the intricacy. For instance, the lattice-work on the reverse of the $100 bill's ''watermelon'' designs could not fool a sharp investigator from the Secret Service. But Ninger's work was so good that it usually went undiscovered until reaching the Treasury. Curiously, Ninger omitted any reference to the Bureau of Engraving and Printing on any of his counterfeits.

Ninger was arrested in 1896, after 14 years of success. His error was laying one of his notes on a wet bar before passing it to the saloon-keeper. The bill's new owner noticed that ink was staining his hands and called the police.

Ninger was sentenced to six years imprisonment. But his trial generated controversy. Supporters urged that Ninger be acquitted because his bills were works of art, indeed works of the then-new Impressionism. And the counterfeit bills fetched many more times their face value as collector's items than as currency. Ninger supported that view.