Reconstruction of a Roman Groma. The Groma or gruma (altered from Greek gnomon "indicator", possibly through Etruscan) was the principal Roman surveying instrument. It comprised a vertical staff with horizontal cross-pieces mounted at right-angles on a bracket. Each cross piece had a plumb line hanging vertically at each end. It was used to survey straight lines and right-angles, thence squares or rectangles.
Early & Rare Example of Paradoxical Illusion This very rare electrum hekte (only 8 known examples) is from the ancient city of Mytilene on the island of Lesbos, circa BC. On the obverse is the head of Athena wearing a crested Corinthian.