LONDON — If you have hearing difficulties, the idea of spending hours watching a play may not be that attractive given the risk of key dialogue slipping from your grasp. The National Theater in London is hoping to change that.

On Wednesday, the theater introduced “smart caption glasses” that display dialogue on the lenses as actors speak. The glasses can be used without charge for the play “War Horse” and for the musical “Hadestown,” and they will be available for all of the theater’s 2019 season.

Marie Pascall, founder of Performance Interpreting, a British provider of sign language interpreters, said in a phone interview that she had recently tried the glasses and had found them “fantastic.” She said they were much better than alternatives like captions above or below the stage, which force you to look away from the action. (The National Theater uses those options for three or four performances per play each season.)

“You get a complete sense of the whole play,” Ms. Pascall said of the glasses. “You’re not having to ping-pong between the actors and words.”