period of revolution of Mars is six hundred and eighty-seven days, and would be less if the mass or weight of our sun was greater. We can hence calculate how many times greater than the mass of the sun must be the mass of the two stars of Mizar for the revolution to be accomplished in one hundred and four days. The result is forty times the mass of the sun. So this little point of light which Mizar in the Great Bear appears to the eye is the equivalent of forty of our suns.
Before the news of this astonishing discovery made at Cambridge had reached Europe, a similar investigation made at the Potsdam Observatory was published. It was directed to the star Algol in the head of Medusa. This star has been known for more than two hundred years to be variable in brightness. It shines for two days and a half with a steady white light, then loses brightness for about four hours and a half, recovers during about four hours and a half, and then continues steady again for two days and a half. The changes go on with great regularity, and it has been believed for the last hundred years that Algol is attended by a double star revolving around it, by which it is concealed from the earth at regular intervals, depending on the period of its revolution. The periodical decrease of brilliancy is similar in its nature and cause to an eclipse of the sun, when the dark moon is interposed between it and the earth. But probable as this belief was, the fact had not been demonstrated. A complete solution has been obtained by spectrum analysis. Prof. Vogel, of the Astrophysical Observatory in Potsdam, and his fellow-worker, Dr. Scheiner, have taken photographs of the spectrum of Algol and carefully measured the dark lines. It has thus been ascertained that these lines move toward the red before the star appears at its weakest, toward the violet after that moment; or, in other words, that Algol is receding from the sun in the first half of its change, approaching it in the second half. This would necessarily occur if the star was describing an orbit around a dark body which should periodically conceal it for a time from our view. The rate of motion of Algol is twenty-three English miles in a second, and its period of revolution is two days, twenty hours, and forty-nine minutes; whence the circumference of its orbit and the distance apart of the centers of the two stars may be computed as was done in the case of Mizar. The latter is found to be less than 3,000,000 English miles, a small enough distance for two so large bodies. From the period of the light-changes and the velocity of the motion we calculate the diameter of the principal star to be 920,000 and of its dark companion 750,000 English miles. The two bodies which form the Algol system are each nearly as large as our sun, the diameter of the sun being taken at 750,000 miles, but their total mass is only about two thirds the mass of