This was the best story I ever read.  I was in maybe the fifth grade, in 1959(?) and it was in a collection of short stories that was in our classroom for when you had finished your work early.  It was not a brand-new book then, either.

But the story!  These two unemployed guys are looking through the want ads, and they find a doctor who wants volunteers for an experiment.  He has a diving board that faces a black window.  The volunteer is supposed to jump from the diving board into that unknown space.  Well, one of the guys does it, but he never comes back.  So his friend returns to the doctor’s office and jumps through it himself.  He describes what it was like:  dim and twilit, with backwards writing on can labels, a sort of wasteland with boulders, and here and there a glowing luminescent ultra-violet hint, not too bright, but noticeable.  Maybe two or three of these lights in the whole landscape.  He figures out that his friend fell in the rocks and was killed, because of the improper alignment of this other world with our own.  I mean, you can’t just jump into it and assume you will be all right.  So he thinks some more, and realizes that the glowing spots are places in our own world.  One of them is the University, and the other appears to be the doctor’s laboratory.  So with great effort he leaps in some way back into the doctor’s laboratory, and then throw the doctor himself through the window into the blackness.

So, does this sound at all familiar?  I’ve never been able to find it again.  I think of it as The Diving Board and the Black Window, but that’s probably not the name of it.

If you can find it, I would be so happy!