As many of you probably know, Superman's heat vision was not an early power. Both Superman and Superboy originally used "the heat" of their X-ray vision to melt things. This changed at some point fairly early in Weisinger's tenure. I am not quite sure why; was it because of the obvious problem that bullets were made of lead and thus not affected by X-rays? So I started going through the early Weisinger Action issues, looking for the changeover. What surprised me at first was how common the "heat of my X-ray vision" meme was in Superman. To give you an idea, it popped up in Action #254,, #255, #256, #258, #259, #262, #263 and #264, or in 8 of 11 issues. But after that it vanished for awhile, only appearing in Action #271. And even that appearance comes with an asterisk: 
But in actuality that particular panel is being narrated by a fake Superman and Luthor henchman named Gypo. So it is far from certain that really represents a real instance of the heat of Superman's X-ray vision being used. On the other hand, it is fairly easy to pinpoint the first use of Superman's heat vision in Action; it appears in Action #275 (April 1961): 
Okay, so in the second appearance of Brainiac, heat vision was established? But you know the problem; there are several other Superman mags to check, so I decided to start walking it back from April 1961, partially to see if there was an earlier appearance and partially to see if there was a more credible last mention of the "heat of my X-ray vision". There was no mention of Superman's heat from eyes power in Superman #144 (April 1961), but I did find this in Superman #143 (February 1961): 
End of story, right? Not quite, however, for when I went forward to see when the first mention of Superman's "heat vision" came, I found mention of the heat of his X-ray vision in stories published after April 1961. The latest mention I can find now is from Superman #146 (July 1961): 
The first mention of heat vision I can find in Superman issues comes from #148 (October 1961): 
The transition from "heat of X-ray vision" to "heat vision" is pretty easy to track in Superboy. From Superboy #87 (March 1961): 
While in the following issue (April 1961): 
So it appears pretty obvious that the changeover came between the issues published in March 1961 and April 1961. Update: This is also one of those areas where you cannot trust the reprints. When I went to read the Superman story in Action #259, the closest issue at hand was the reprint in 80-Page Giant #1 (August 1964), where this sequence occurs: 
But after noticing that there were several mentions of "heat of my X-ray vision" after this, I hunted down the original, and sure enough: 
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