THE HEAVENS IN MAY 



343 



moved far to the east of the Hyades, 

 almost to the border of the great con- 

 stellation of the Bull. 



TIlE may stars. 



In the west we now see the very 

 brilliant groups of ( >rion and Taurus 

 and the far fainter Perseus with its re- 

 markable Demon Star. Algol, all just 

 sinking below the ground. The bright 

 Dog Star has wholly disappeared, 

 while the lesser Dog Star, the bright 

 twins and the brilliant golden sun, Ca- 

 pella, are very near the borders of our 

 evening sky. The great Leo and the 

 Bear are still high in the heavens, but 

 these too are beginning their declension 

 toward the west, while due south the 

 Water Snake stretches its great length 

 almost across the entire heavens. 



Meanwhile, in the east, we see many 

 beautiful summer groups steadily 

 mounting upward to take the place of 

 those that are leaving us. The very 

 large Bootes with its brilliant, golden- 

 yellow Arcturus, the delicate Northern 

 Crown, the interesting Hercules with 

 its wonderful cluster of suns, have ail 

 mounted high up from the ground, 

 while below these we see that the 

 Northern Cross and the great summer 

 groups of the Serpent and the Serpent- 

 Holder are now beginning to enter the 

 evening sky. 



In the southeast, will be found the 

 Scorpion, the most striking of all the 

 summer groups, although as yet only 

 the claws and front part of this creature 

 have emerged from below the ground. 

 Soon its fiery-red star, Antares, will ap- 

 pear, and when the whole becomes vis- 

 ible we will see that the string lies al- 

 most on the center of the wonderful 

 summer branch of the Milky Way, and 

 when, as now, the winter branch is 

 sinking below the western horizon, the 

 summer branch begins to rise above the 

 ground in the east. 



To the right of Scorpio are the two 

 stars of the Balances, and westward 

 from these there stretches the great 

 Harvest constellation of the Virgin, 

 whose bluish-white star, Spica, cele- 

 brated from the earliest times as the 

 Solitary or Defenceless One, is the 

 brightest object in this part of the 

 heavens. This distant sun belongs to a 

 remarkable class of stars, each member 

 of which, though it appears as a single 

 star even in the largest telescopes, is 



known to be in revolution about a 

 darker, invisible companion. The reve- 

 lation of these invisible stars about 

 which many of the bright suns are re- 

 volving is one of the most surprising 

 of very modern discoveries of astron- 

 omy. 



HOW INVISIBLE STARS ARK FOUND. 



It is well known to everyone that 

 when a beam of white sunlight shines 

 through a triangular prism it emerges 

 as a beautiful band of light whose col- 

 ors blend from violet at one end to 

 red at the other. This is because the 

 apparently simple beam of white light 



Figure 2. The formations of the spectrum of 

 sunlight. 



is actually made up of almost an infi- 

 nity of light beams of different wave 

 lengths, the waves known to us as vio- 

 let light being the shortest while those 

 which produce upon our eyes the im- 

 pression of a red color are the longest. 

 The shorter the light waves in the 

 ether are the more they will be bent 

 out of their course by their passage 

 through the prism. As the wave 

 lengths of violet light are less than 

 those of red light the colors will thus 

 be separated from one another as il- 

 lustrated in Figure 2. 



If a narrow slit be placed before the 

 prism so that only a very thin beam of 

 sunlight can shine through it, the color 

 strip will at once become crossed by a 

 multitude of fine, black lines whose po- 

 sition with reference to the direction 

 of the narrow beam can be measured 

 with great accuracy. From many of 

 the stars, of which Capella is an ex- 

 ample, the color strip, or Spectrum, is 

 exactly like that made by the light of 

 our own sun ; from others we obtain a 

 spectrum crossed by bright lines in- 

 stead of dark ones. But in either case 

 the position of the lines depends upon 



