One of the most interesting problems of the astronomer at present is whether the motions of the heavenly bodies, as determined by our most refined methods of observation, go on in rigorous accordance with the law of gravitation. … In 1845 Le Verrier found that the centennial motion of the perihelion of Mercury derived from observation was greater by 35″ than it should be from the gravitation of other planets, and his result has been more than confirmed by subsequent investigations, the most recent discussion of observations showing the excess of motion to be 43″ per century. In this case there can be no doubt as to the correctness of the theoretical result, since the computation of the secular motion of the perihelion is a comparatively simple process. It follows that either Mercury must be acted upon by some unknown body or the theory of gravitation needs modification.