Maybe it's easier to list the things you can't copyright.

Hubcaps. You can't copyright hubcaps.

There is even one of those at the bright new exhibit at the Library of Congress. Strung along the fourth-floor corridor at the Madison Building, this show, "By Securing to Authors," covers the high spots of copyright law in America.

It is also fun.

You get to listen to bits from two pop songs of the '20s, "Dardanella" and Jerome Kern's "Kalua." Kern was accused of lifting an eight-note figure in the accompaniment, and you can hear it there, not quite plain as day (in fact the "Dardanella" people must have had extremely sharp ears as well as litigious instincts). Nobody thought the great Kern had intentionally copied the phrase, but as the judge said, unconscious copying is no defense to infringement.

A quick clip from "The Maltese Falcon" -- plus the Falcon itself -- enlivens another court case. Warner Bros. had bought the film rights to Dashiell Hammett's novel, and when the author subsequently wrote another Sam Spade adventure, the studio, believing it now owned Sam Spade himself, sued.

This seems an astoundingly stupid case of killing the golden goose (did they think their own hacks could do Sam Spade?), and sure enough, a landmark decision established that the character, "the chessman in the game of telling the story," still belonged to Hammett.

The Falcon, though fully as black and sleek and sinister as one remembers, does seem awfully small, hardly 10 inches high. One remembers the ship captain -- played by director John Huston's famous father Walter in an uncredited bit -- staggering into Spade's office with a package as big as a baby.

There is also a sample of the video game "Defender," subject of another big court case. The video exhibits were still being tuned up on opening day yesterday.

A recurring problem over the years has been the proliferation of media far beyond the maps, charts and books specified in the original federal act of 1790. One startler was Napoleon Sarony's photograph of Oscar Wilde, taken in 1884. The court decided that the Founding Fathers would have included photos if they had known they were going to be invented.

The Wilde photo is here, as is a vintage circus poster ("The Stirk Family . . . World's Greatest Bicyclists"), which led to a celebrated dictum by Supreme Court Justice Oliver Wendell Holmes that questions of artistic merit should not be left to lawyers.

Much, much later comes the issue of copyrighting computer programs. They are indeed copyrightable, the Copyright Office judged in 1964 after four years of study, but must be presented in "human-readable form," a comforting thought.

Some of the stormy early history of copyrighting has been left out of the exhibit, notably the uproar over the fact that the original act failed to cover foreign authors. Dickens and other English writers were pirated so freely in this country that American authors were being neglected. The Chace Act of 1891 finally ended the controversy. The 1870 law setting up a copyright office is mentioned, but not much is said about the fact that the ensuing avalanche of material more or less caused the building of a new Library of Congress.

But such details will hardly be missed in this colorful reminder of just how much the copyright principle means to all sorts of artists. And how excruciatingly subtle a line must be drawn.

Did the movie "The Cohens and the Kellys" infringe on the Broadway hit "Abie's Irish Rose"? No. Did the film "Letty Lyndon" infringe on the play "Dishonored Lady"? Yes. ("No plagiarist can excuse the wrong by showing how much of his work he did not pirate," thundered Judge Learned Hand.) Can the city of Chicago demand royalties for small copies of Picasso's giant civic center sculpture? No. Can a sculpture designed from the start to be a lamp base be copyrighted? Yes.

There is so much sheer stuff here that you think you are at the Smithsonian: posters and stills from the nine different versions of Edward Knoblock's 1911 play "Kismet"; Mickey Mouse Big Little Books; "Star Wars" masks; Tarzan coloring books; "Gone With the Wind" dinner plates; watches; games; dolls; T-shirts; buttons; and pictures of the pedestal for the Statue of Liberty, the ultimate spinoff.

There is Copyright No. 159, for the Atlantic Monthly of August 1870, including an article about the English governess at the Siamese court. And the 1887 papers for Coca-Cola (J.S. Pemberton, chemist, sole proprietor, Atlanta): "syrup and extract for soda water and other carbonated beverages, not only a delicious, exhilarating, refreshing and invigorating Beverage . . . but a valuable Brain Tonic and a cure for all nervous affections, Sick Head-ache, Neuralgia, Hysteria, Melancholia, etc."

And hubcaps. For your information, a hubcap that looks like a wire-spoked wheel cannot be copyrighted.