A spyglass is a small telescope you hold in your hand — the kind that slides open in sections and that you hold up to one eye. Sailors used them in the old days to look at far-off ships and coastlines. The word belongs to the past. Nobody buying one today would call it that; they would say telescope, or for the two-eyed kind, binoculars. So you meet spyglass in pirate stories, sea novels, history writing, and films set long ago — and that old feeling is exactly why writers still choose it. Note that although the word starts with spy, the tool is not really for spying on people; it is simply for seeing far.