Helping someone else find their book: this story is about two unemployed men who volunteer for a scientific experiment. It requires them to jump from a diving board through a black window, which brings them to a different dimension. One of the men dies after a bad dive, but the other survives the jump and explores this other world before eventually finding his way back and ultimately tossing the scientist (who is conducting the experiment) out the black window.
The book was part of an anthology that the reader found in his 5th grade classroom in the late 1950s.
Thanks for any insights.