Al-Hitham's Billiard Problem

Al-Hitham set out to verify the law of reflection. He used a semicircular block with holes drilled around its edges. Light was projected through these holes and reflected from a mirror placed along the diameter. The experiment demonstrates that light is only visible if it passes through the hole when it is at an equal angle about the central point. However, unlike Ptolemy, he doesn't provide us with results. This is strange, as the apparatus he employed is described in precise detail, with measurements for its construction given to the equivalent of millimeters. Undoubtedly, his setup would have been capable of verifying the law, though perhaps his desire for extreme precision is the problem.