Eddington's comparator. This device was built by Sir Arthur Eddington (1882-1944), and was designed for measuring changes in the positions of stars, seen on telescope glass plates mounted below the movable microscopes. Eddington would have used a comparator like this to study plates from the 1919 eclipse expeditions, designed to test Einstein's 1915 theory of General Relativity. This predicted that light passing close to a large mass (like the Sun) bends twice as far as predicted by Newton's theory of gravity. This lensing effect changes the position of stars seen near the Sun during a total solar eclipse. Eddington's results, now considered inconclusive, were said to show the shift and were hailed as support for General Relativity.