James Blish 
Mistake Inside 

One minute Dr. Hugh Tracy, astronomer, had been charging at the door of Jeremy Wright’s apartment, an automatic in his hand and blind rage in his heart. As his shoulder had splintered the panel, the world had revolved once around him, like a scene-changing stunt in the movies. Then he was not standing in Jeremy Wright’s apartment at all, but in a low-roofed, dirt-floored room built of crudely shaped logs, furnished only with two antique chairs and a rickety table from which two startled men were arising. The two were dressed in leathern jerkins of a type fashionable in the early 1700s.