that it is impossible for a star to run along a straight line for a certain distance, stop, turn back, again retrace its movement, stop, and again return. Such movement is simply forbidden by the laws of motion. We can, however, easily ascertain that there is a type of motion possible for Algol which shall be compatible with the results of the spectroscopic research and also be permitted by the laws of motion. There is no objection to the supposition that Algol is moving in a path which is nearly, if not exactly, a circle. In this case it would only be moving as does the moon, or the earth, or any of the other planets. It will be only necessary to suppose that the plane of the orbit of Algol is directed nearly edgewise towards us. During the description of one semicircle Algol will be coming towards us, while, during the other semicircle it will be going from us, and thus the observed facts of the movement are reconciled with the laws of motion. Of course, this involves a certain periodic shift in the position of Algol in the heavens. It must, for instance, when moving most rapidly from us be at a distance equal to the diameter of the circle from the position which it has when moving most rapidly towards us. This is true, but the extent of the shift of place is far too small to be visible to our instruments. In fact, it can be shown that the visual size of the circle in which Algol revolves could hardly be larger than is that which the rim of a three-penny bit would appear to have if viewed from a situation five hundred miles away. It is one of the extraordinary characteristics of the spectroscopic method that it renders such an orbital movement perceptible.
The fact that Algol revolves in an orbit having been thus demonstrated, we can again call in the assistance of
