364 THE STELLAR UNIVERSE. 



guish it. The large star is white, and the small star somewhat of a ruddy ap- 

 pearance, and is distant from the larger seventeen seconds, or about three or 

 four of its diameters. 



Fig. 8 is the double star Castor. The smaller star is nearly half the size 

 of the larger, and they are distant about five seconds, or two diameters of the 

 principal star. They are both of a whitish color. Castor and Pollux lie to 

 the northwest of Orion, at a considerable distance from it. They are very 

 conspicuous, are within five degrees of each other, and rise to a very high 

 elevation when passing the meridian, and may be seen throughout the whole 

 winter and spring months. Castor is the more elevated of the two. 



Fig. 9 represents Rigel, a splendid star in the left foot of Orion. The small i, 

 star is a mere point, and very difficult to be distinguished, and is three or four ; 

 diameters of the large star from it. The large star is white, the small one of < 

 a reddish hue. 



Fig. 10 shows the double star Castor, with a magnifying power of 300. It 

 likewise shows the angular position of the small star at the present time in 

 respect to Pollux (fig. 11), by which it appears that it is nearly at a right angle 

 to a line joining Castor and Pollux, whereas in the time of Dr. Bradley it was 

 parallel with a line joining these two stars. 



Figs. 12, 13, 14, and 15, exhibit views of the double star f Bootis, with four 

 magnifying powers. Fig. 12 is its appearance with a power of 227; fig. 13, 

 with a power of 460 ; fig. 14, with a power of 900 ; and fig. 15, with a power 

 of 1,100. 



Figs. 16, 17, and 18, represent telescopic views of the triple star in the left 

 fore foot of the constellation Monoceros, or the Unicom, which forms a very 

 beautiful object in this class of stars. This star appears at first double, but 

 with some attention one of the two is discovered to be also double ; the first 

 of them is the largest. The color of all these stars is white. With a small 

 power they appear as in fig. 16 ; with a power of 220, as in fig. 17 ; and with 

 a power of 450, as in fig. 18. There is a beautiful object of this description, 

 but somewhat different in the configuration of the three stars of which it is 

 composed, to be seen in the tail of the Great Bear ; it is the star Ursa, called 

 also Mizar, and is the middle star in the tail. 



All the objects here enumerated may be seen with a good three and a half 

 feet achromatic telescope, with an object-glass of 2f inch diameter. The 

 double star Castor may be seen with so low a power as 80, but more distinctly 

 with higher magnifiers. 



Fig. 19. 



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 composing a double star, being situate very nearly in the same 

 direction 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. 19, 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 



