I’m trying to find a science fiction short story I read in the late 70s/early 80s, possibly in an older anthology: it was likely written in the 50s or 60s.  

It’s about time travel and its limits: basically, humanity has found a way to **travel backwards in time but not physically, just as onlookers**.  

So it’s **used in forensics to travel back to when a crime has been committed and see who the felon is**: I remember one of the characters says that if you’ve been murdered they cannot bring you back to life but at least they have the means to capture your murderer.  

I think they’re using kind of floating balls with built-in cameras for the task but I’m not sure about this, it could be from another story.