Previously unreleased footage shows the conductor and composer Leonard Bernstein leading one of his final rehearsals. Fittingly, it shows him teaching a Copland symphony to students at Tanglewood, the Boston Symphony Orchestra’s summer home in the Berkshires, in 1990 — 50 years after he had studied there with Copland.

In the rehearsal, which he led just two months before his death, at 72, Mr. Bernstein urges the young players of the Tanglewood Music Center Orchestra to loosen up as they play Copland’s Third Symphony, telling them to imagine themselves “standing on the sidewalk, thumbs in your jeans, whistling casually.” Seated, he looks quite weak but fully engaged with the music.