ADDRESS. 29 



The K line in the photographs of Mizar, taken at the Harvard Col- 

 lege Observatory, was found to be double at intervals of fifty-two days. 

 The spectrum was therefore not due to a single source of light, but to 

 the combined effect of two stars moving periodically in opposite direc- 

 tions in the line of sight. It is obvious that if two stars revolve round 

 their common centre of gravity in a plane not perpendicular to the line 

 of si^ht, all the lines in a spectrum common to the two stars will appear 

 alternately single or double. 



In the case of Mizar and the other stars to be mentioned, the spec- 

 troscopic observations are not as yet extended enough to furnish more 

 than an approximate determination of the elements of tbeir orbits. 



Mizar especially, on account of its relatively long period, about 105 

 days, needs further observations. The two stars are moving each, with a 

 velocity of about fifty miles a second, probably in elliptical orbits, and 

 are about 143 millions of miles apart. The stars of about equal bright- 

 ness have together a mass about forty times as great as that of our sun. 



A similar doubling of the lines showed itself in the Harvard pboto- 

 graphs of yS Aurigse at the remarkably close interval of almost exactly 

 two days, indicating a period of revolution of about four days. Accord- 

 ing to Vogel's later observations, eacb star has a velocity of nearly seventy 

 miles a second, the distance between the stars being little more than 

 seven and a half millions of miles, and the mass of the system 4' 7 times 

 that of the sun. The system is approaching us at the speed of about 

 sixteen miles a second. 



The telescope could never have revealed to us double stars of this 

 order. In the case of /3 Auriga3, combining Vogel's distance with 

 Pritchard's recent determination of the star's parallax, the greatest 

 angular separation of the stars as seen from the earth would be l-200th 

 part of a second of arc, and therefore very far too small for detection 

 by the largest telescopes. If we take the relation of aperture to sepa- 

 rating power usually accepted, an object glass of about eighty feet in 

 diameter would be needed to resolve this binary star. The spectroscope, 

 which takes no note of distance, magnifies, so to speak, this minute 

 angular separation 4,000 times ; in other words, the doubling of the 

 lines, which is the phenomenon that we have to observe, amounts to the 

 easily measurable quantity of twenty seconds of arc. 



There were known, indeed, variable stars of short period, which it 

 had been suggested might be explained on the hypothesis of a dark 

 body revolving about a bright sun in a few days, but this theory was 

 met by the objection that no sacli systems of closely revolving suns were 

 known to exist. 



The Harvard photographs of which we have been speaking were 

 taken with a slitless form of spectroscope, the prisms being placed, as 

 originally by Fraunhofer, before the object glass of the telescope. This 

 method, though it possesses some advantages, has the serious drawback 



