By observations with the Potsdam spectograph, Professor Vogel found that the bright star of Algol pulsated backward and forward in the visual direction in a period corresponding to the known variation of its light. The explanation which had been suggested for the star's variability, that it was partially eclipsed at regular intervals of 68.8 hours by a dark companion large enough to cut off nearly five-sixths of its light, was, therefore, the true one. The dark companion, no longer able to hide itself by its obscureness, was brought out into the light of direct observation by means of its gravitational effects. Seventeen hours before minimum Algol was receding at the rate of about 241/2 miles a second, while seventeen hours after minimum it was found to be approaching with a speed of about 281/2 miles. From these data, together with those of the variation of its light, Vogel found, on the assumption that both stars have the same density, that the companion, nearly as large as the sun, but with about one-fourth his mass, revolved with a velocity of about fifty-five miles a second. The bright star of about twice the size and mass moved about the common center of gravity with the speed of about 26 miles a second. The system of the two stars, which were about 31/4 millions of miles apart, considered as a whole, was approaching us with a velocity of 2.4 miles a second. The great difference in luminosity of the two stars, not less than fifty times, suggested rather that they were in different stages of condensation, and dissimilar in density. It was obvious that if the orbit of a star with an obscure companion was inclined to the line of sight, the companion would pass above or below the bright star and produce no variation of its light. Such systems might be numerous in the heavens. In Vogel's photographs, Spica, which was not variable, by a small shifting of its lines revealed a backward and forward periodical pulsation due to orbital motion. As the pair whirled round their common center of gravity, the bright star was sometimes advancing, at others receding. They revolved in about four days, each star moving with a velocity of about 56 miles a second in an orbit probably nearly circular, and possessed a combined mass of rather more than two and one-half times that of the sun. Taking the most probable value for the star's parallax, the greatest angular separation of the stars would be far too small to be detected with the most powerful telescopes. THE VALUE OF PHOTOGRAPHY.