TRIPLE STARS. 



colour, so that the complement of the whole light of the fainter 

 star finds the retina more sensible than that part which is iden- 

 tical in colour with the brighter star, and the impression of the 

 complementary colour accordingly prevails. In many cases, how- 

 ever, the difference of colour of the two stars is real. 



When the colours are complementary, the more brilliant star is 

 generally of a bright red or orange colour, the smaller appearing 

 blueish or greenish. The double stars i Cancri and 7 Andromacha) 

 are examples of this. According to Sir J. Herschel, insulated stars 

 of a red colour, some almost blood-red, occur in many parts of the 

 heavens ; but no example has been met with of a decidedly green 

 or blue star unassociated with a much brighter companion. 



55. When telescopes of the greatest efficiency are 1 directed upon, 

 some stars, which to more ordinary instruments appear only double, 

 they prove to consist of three or more stars. In some cases one 

 of the two companions only is double, so that the entire com- 

 bination is triple. In others, both are double, the whole being, 

 therefore, a quadruple star. An example of this "latter class is 

 presented by the star e Lyrre. Sometimes the third star is much 

 smaller than the principal ones, for example, in the cases 

 of C Cancri, | Scorpii, 11 Monocerotis, and 12 Lyneis. In others, 

 as in 6 Orionis, the four component stars are all conspicuous. 



56. When the attention of astronomers was first attracted to 

 double stars, it was thought they would afford a most promising 

 means of determining the annual parallax, and thereby discovering 

 the distance of the stars. If we suppose the two individuals com- 

 posing a double star, being situate very nearly in the same direc- 

 tion as seen from the earth, to be at very different distances, it 

 might be expected that their apparent relative position would vary 

 at different seasons of the year, by reason of the change of position 

 of the earth. 



Let A and B, fig. 6, represent the two individuals composing a 

 double star. Let c and D represent two positions of the earth in 

 its annual orbit, separated by an interval of half a year, and placed 

 therefore on opposite sides of the sun s. When viewed from c, 

 the star B will be to the left of the star A ; and when viewed from 

 D, it will be to the right of it. During the intermediate six 

 months the relative change of position would gradually be effected, 

 and the one star would thus appear either to revolve annually 

 round the other, or would oscillate semi-annually from side to side 

 of the other. The extent of its play compared with the diameter 

 c D of the earth's orbit, would supply the data necessary to deter- 

 mine the proportion which the distance of the stars would bear to 

 that diameter. 



The great problem of the stellar parallax seemed thus to be 



207 



