Movies for television are getting better. Most still tend to be about beautiful people injured in auto wrecks or skiing tumbles and climbing back to life on the rope ladder of a love affair. But now and then, a director with some sense of timing and composition sneaks onto the air with a true beaut.

"Ishi, the Last of His Tribe," on NBC tonight at 9 on Channel 4, may frighten off mainstream TV viewers with its hushed austerity and verbal economy, but those who stick with the show will see something extraordinary-haunting and artful.

The script, based on a sociological classic by Theodora Kroeber, was begun by the late Dalton Trumbo and completed by his son, Christopher, and directed with both understatement and imagination by Robert Ellis Miller. It's the story of an Indian found running in the expiring wilderness of California in 1911 and how he became a living casebook of the American past for the anthropologist who befriended him.

Dennis Weaver plays the anthropologist, Alfred Kroeber, with his usual grass-roots credibility, though with less forced colloquialism than one might expect. The most extraordinary moments in the film, however, are dominated by Joseph Running Fox as the adolescent Ishi and Devon Ericson as hie cousin.In flashbacks, their already tiny tribe, the Yahi, are reduced steadily in number until they alone remain. The inevitability of this is as certain and as heartbreaking as the invasion of the land by the machine, and the imagery employed to make to point is clean and direct.

After the melodramatic excesses of guilt-mongering theatrical films like "Little Big Man," the comparatively dispassionate approach of "Ishi" comes across as clarifying anc intelligent. There is, of course, romanticization of Indian ways and mysticism, and language tends to be high-blown when Indians speak it. Thus it is said of one who died that he "became dead."

The cinematography, by Woddy Omens, is striking enough in its view of the country to mitigate miscalculations in the scrip, however. As the grown Ishi, Eloy Phil Casados at one point confronts icons of earlier civilizations displayed in Kroeber's musuem; this triggers one of many flashbacks to his own earlier time, some of them beautifully realized reveries of pre-industrial life.

"ishi" seems finally and arrestingly a statement - on ancestry, and progress, and earth's ethnic diversity - in a field dominated by, at best, incomplete sentences. It will attract a relatively small bu generously rewarded national audience tonight.