If you were a young woman between 1930 and 1960, there was apparently a worrying non-zero chance you’d end up as an experiment inside a giant glass tube.
As a visual trope, the glass tube is both economical and efficient. It communicates multiple symbols at once. First is threat. The woman is imprisoned — but she is not imprisoned in a cage dangling over a pit of hungry alligators or strapped to a table about to be sawed in half. The glass tube evokes the imagery of science. She is not the captive of the mad brute — usually a foreigner trying to steal our women! — but is instead the toy of the mad scientist, who is intellectualism gone awry.
(Alternatively, it may be an alien, who is the mad brute/foreigner in sci-fi form. They are invasion/decrepitude from without. Intellectualism represents invasion/decrepitude from within. Either way, only the potent virility of the hero, the antidote to both, can save her.)
The tube also resembles a display case, making it clear, probably more than any other visual trope, that the young woman is a sexual prize and the story inside the magazine is a parable about sexual access to the females of the herd.
The glass tube is related to the earlier trope of the glass coffin, inside of which the virginal maiden, Snow White, is preserved in death until reborn by her lover’s first kiss — symbolic of course of the sex act. Despite that she is the protagonist of her story, it’s only by anointment with a penis that she becomes a woman.