O Ovo De Cristal by H. G. Wells

A London antique dealer acquires an unusual crystal egg that, when gazed into, reveals moving landscapes and distant activity on a red, canal-crossed planet; the dealer and a curious acquaintance watch the scenes night after night, coming to see the object as more than ornament—a remote viewing device through which intelligent beings on that world may be observing Earth—and the discovery leaves them with an unsettling sense of possible contact and consequence.