The premise of this tale is that a scientist, or small group of scientists, have created a device that will allow them to open a viewing portal to the past. With it, the operator can watch events unfold from a given time/place through the created portal. The portal is one way, so the observed cannot see the observers. The one quirk in the design is that when it is activated, it creates a glowing outline of the where the portal has manifested in the past. Both as it forms, and for a time afterwards, IIRC.
The bulk of the story details with the scientists (possibly also historians of a sort?) observing the construction of Stonehenge. There are passages about the stone being hauled up, etc. I don't recall if the viewing was time-lapse or real time. As the viewing ends, the scientists in the present consider the entire experiment to be a resounding success and an historical breakthrough.
It is then we get the story's twist. One of the other scientists suddenly raises a concern with the lead scientists. Since the glowing effect of the portal would have been visible well-before they began viewing the construction of Stonehenge, and probably would have been seen as magical or unearthly, what if their own experiment was the inspiration for Stonehenge? (There is a term for this sort of effect, I believe, but I cannot recall what it is).
I read this one in the 1980s, as usual. I'm almost positive that it was part of an anthology in my dad's collection. I'd guess 1970's or earlier for the story's publication date.