Superman is investigating a strange rash of disappearing objects and discovers that they were all made from steel produced in a certain factory. Sensibly, he goes there to check things out.
The next logical step, of course, is for Superman to examine the molecular structure of the steel to see if anything hinky is going on. Now, I have to assume that someone on the creative team either just plain had a neat idea or felt sorry leaving the rumpled old factory owner out of the action, because instead of just showing us what Superman sees in a POV panel, they had him make this startling revelation:
AND THEN IT WORKS!
Oh how I wish that this was something that ever came up again (not that I've read every Superman comic extant, but I have to assume I would have heard of it by now otherwise). I can only imagine panel after panel of Lois Lane or Jimmy Olsen staring at Superman's eyes through magnifying glasses, or Lois trying to take pictures of Clark's eyeballs to see if he was using vision-based powers while pretending to cower in fear. Ah, well. A man can dream.
From World's Finest Comics No. 48.