dipleidoscope

from The Century Dictionary.

• noun An instrument for indicating the passage of the sun or a star over the meridian by the coincidence of two images of the object, the one formed by single and the other by double reflection.

from the GNU version of the Collaborative International Dictionary of English.

• noun (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun astronomy An instrument for determining the time of apparent noon. It consists of two mirrors and a plane glass 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 centre is on the meridian.