For the Smithsonian, the trick is to find a balance between public and private activities: to encourage donors, for example, without forgetting that something called the National Gallery requires a collective sensibility, too, or to attract good business managers without losing good curators. Sometimes the institution loses that balance -- as it did when it recently signed a controversial contract with Showtime Networks, a commercial cable channel, to produce a series called "Smithsonian on Demand." In part because the contract details are confidential and in part because the Smithsonian says the deal gives Showtime Networks the right of first refusal on any documentary that makes more than "incidental" use of Smithsonian collections, the deal has angered documentary filmmakers who have traditionally enjoyed open access -- for a small fee -- to museum collections.