THE DISCOVERY OF INVISIBLE WORLDS. 503 



est hopes. With, this explanation we are prepared to understand 

 the important discoveries that have been made at Potsdam and 

 Cambridge. 



The bright star Mizar in the Great Bear is known to all. It is 

 resolved in the telescope into two stars, the bright star being ac- 

 companied by a dimmer one, which is evidently a satellite, but pos- 

 sesses a period of revolution of about two thousand years. The 

 spectrum of the principal star has been photographed several times 

 since 1887 at the Cambridge Observatory, Mass., and the photo- 

 graphs have been carefully studied by Miss A. C. Maury, a niece of 

 the celebrated Dr. Draper. The curious fact has been brought out 

 that one of the photographed dark lines appears, at times, as if it 

 was split into two fine lines. The doubling appears in the photo- 

 graphs of May 29, 1887, and of May 17 and 27, and August 28, 1889. 

 In other photographs the lines appear washed out, as if they con- 

 sisted of two lines, yet not quite separated ; while on still others 

 they appear clearly defined. On making up the registers of 

 the times when the lines presented their different appearances, 

 it was found that they appeared double at intervals of fifty- 

 two days, washed out a few days before and afterward, and 

 at other times single and sharp. By way of test the time was 

 predicted when they should appear double again, and they came 

 so, true to the forecast. The other lines in the spectrum of 

 Mizar are not very sharp, and some of them are very faint. Care- 

 ful examinations have shown that those few sharp lines also ap- 

 pear somewhat washed and broader when the first line is doubled, 

 while the faint lines are at the same time very hard to see. The 

 explanation of these variations, according to Prof. Pickering, Di- 

 rector of the Cambridge Observatory, lies in the supposition that 

 the chief star Mizar is itself a double star, whose components re- 

 volve around one another in one hundred and four days, but are 

 still so close together that no telescope can separate them. They 

 appear even in the most powerful telescope only as a single round 

 star. When one of the two stars is moving toward the earth, all the 

 lines in its spectrum are pushed toward the blue end ; at the same 

 time the second star, since both participate in the revolution, must 

 be receding from the earth, and the lines of its spectrum are pushed 

 toward the red end. As soon, again, as the motion of the stars is 

 perpendicular to a line drawn to the earth, all the lines will have 

 their normal position, and mutually cover one another ; they will 

 appear single and distinct. The amount of the motion is calcu- 

 lated, from the extent of the doubling, at a hundred English miles 

 in a second ; from the period of revolution of one hundred and four 

 days, the circumference of the orbit is deduced to be 900,000,000 

 English miles, and the distance of the two stars apart 143,000,000 

 miles, or about the distauce of the planet Mars from the sun. The 



