It’s that old "the obscuring edge of the transparent vat trick,"
coupled with a strategically placed whiff of smoke.

They couldn’t paint them completely naked, so artists for older pulp magazines deceived the viewer by artistically suggesting that these female victims of mad scientists were nude, while blocking any view of their nudity. (Click any of the pics to enlarge.)

Why did the mad scientist place his cloth right there? 
From the pulp magazine "New Mystery Adventures."

What is this naked woman doing in a tube? 
How did they design the tube so it completely 
covers all of her private parts?
Is the gentleman in the foreground
examining her with an airport 
Full Body Scanner?

The naked woman is actually illustrating 
the story "Newscast" by Harl Vincent, 
from "Marvel Science Stories."

Notice that both her naked human body and the 
transparent sheet are defying the laws of gravity.
From "Fantastic Science Fiction," 1956.

Another mad scientist’s apparatus saves us from
viewing a nipple in this cover from "Horror Stories."
I see that the artist got lazy and just painted out 
any naughty bits below her waist.

"Quickly, Igor, position the girl in the jar 
so our viewers will not see anything nasty…"

An old dodge – using a naked person’s own arm 
to cover her nakedness. From "Amazing Stories."

The fluoroscope x-ray machine will 
hide her breasts yet reveal the jewel!

What is it about naked gals in jars? Fumes, smoke, 
ridges and reflections all keep this image PG.

For more science fiction, mad scientist and other pulp, see:
 – It’s dark in the dark>>
– Cover Browser>>
– Mad Scientists>>
– Mad Science, PBase>>