n. [ Gr. &unr_; double + &unr_; image + -scope. ] (Astron.) An instrument for determining the time of apparent noon. It consists of two mirrors and a plane glass disposed in the form of a prism, so that, by the reflections of the sun's rays from their surfaces, two images are presented to the eye, moving in opposite directions, and coinciding at the instant the sun's center is on the meridian. [ 1913 Webster ]