its brightness. The whole time for the decrease and the restoration of light is about ten hours. The form of this light-curve points unmistakably to the eclipse of a bright star by a relatively dark companion. This explanation, first proposed by Goodricke, was developed by Pickering, and proved spectroscopically by Vogel. Dr. Alexander W. Roberts, of Lovedale, South Africa, has recently developed a method for determining the absolute dimensions of an Algol binary. The theory which underlies the determination is that light takes an appreciable interval of time to traverse the orbit of a binary system. For an accurate solution observations of the highest precision are essential. Precise photometric observations of such objects have been made by Professor O. C. Wendell, of the Harvard Observatory. The cause of variation is in general the same for all the Algol variables, though there are minor differences of importance. As might be expected, they show great regularity. Nevertheless, there are certain secular variations from causes not well understood. The period of Algol is believed to vary slightly, and Dr. Chandler explains this as due to the presence of a third body. M. Tisserand, however, has advanced a different theory. He assumes a slight flattening of the globe of Algol, and an elliptical orbit for the companion. These rival theories can be settled only by elaborate determinations of the light-curve during many years. According to Dr. Chase, Algol is at a distance of ninety-three light-years. Vogel finds the diameter of each of the components to be nearly a million miles, and the distance between them little more than three million miles. There are doubtless thousands of binary systems in the heavens, one component of which is more or less obscure. Such a system, and it holds true even if the components are equally luminous, becomes for us an Algol variable when the plane of revolution passes through or near the earth. Such systems are comparatively rare. At the present time only thirty-eight are known. The largest variation yet found is that of Fleming's Algol, R. W. Tauri, whose light at minimum is only one twenty-sixth as great as its usual amount. It would be possible for a dark companion of the same size as the bright component to completely eclipse it. In the case of ν Cephei, indeed, this probably takes place, so that the light while the eclipse lasts comes entirely from the dark companion. The companion is only relatively dark, however, so that its light alone is about one eighth as great as the combined light of both components. If the companion, in such a case, were completely obscure, there would be a total eclipse of the star's light, but no such case has yet been found.
