Ivory jealousy glas with fishskin case, c 1750.
© Science Museum / Science & Society Picture Library
A jealousy glas or polemoscope, to give its technical name, is a modified telescope fitted with a diagonal mirror to view sideways. They were made to look like an opera (prospect) glas, a low power telescope popular with theatregoers wishing to see the performance on the distant stage more clearly. Used by the gentry from the 18th century, the jealousy glas was employed to spy upon the audience while appearing to be looking ahead at the stage. This example is made of dark ivory with a shagreen (sharkskin) case.