NOTES. 505 



Six months afterwards, the earth having moved through half of its orbit 

 would arrive at B, and then the star would appear in the direction BS', if 

 the diameter AB, as seen from S', had any sensible magnitude. But AB, 

 which is 190,000,000 of miles, does not appear to be greater than the thickness 

 of a spider's thread, as seen from 61 Cygni, supposed to be the nearest of the 

 fixed stars. 



NOTE 224, p. 430. The mass is found in the manner explained in note 133; 

 but the method of computing the distance of the star may be made more clear 

 by what follows. Though the orbit of the satellite star is really and apparently 

 elliptical, let it be represented by CDO, fig. 14, for the sake of illustration, 

 the earth being in d. It is clear that, when the star moves through CDO, 

 its light will take longer in coming to the earth from O than from C, by the 

 whole time it employs in passing through O C, the breadth of its orbit. When 

 that time is known by observation, reduced to seconds, and multiplied by 

 190,000, which is the number of miles light darts through in a second, the pro- 

 duct will be the breadth of the orbit in miles. From this the dimensions of the 

 ellipse will be obtained by the aid of observation, the length and position of 

 any diameter as Sp may be found; and as all the angles of the triangle dSp 

 can be determined by observation, the distance of the star from the earth may 

 be computed. 



NOTE 225, p. 438. One of the globular clusters mentioned in the text is re- 

 presented in fig. 1, plate 5. The stars are gradually condensed towards the 

 centre, where they run together into a blaze somewhat like a snowball. The 

 more condensed part is projected on a ground of irregularly scattered stars, 

 which fills the whole field of the telescope. There are few stars in the neigh- 

 bourhood of this cluster. 



NOTK 226, p. 435. Fig. 2, plate 5, represents one of those enormous rings in 

 its oblique position. It has a dark space in the centre, with a small star at 

 each extremity. 



NOTE 227, p. 435. Fig. 3, plate 5, may convey some idea of the ring in the 

 constellation of the Lyre mentioned in. the text. 



NOTE 228, p. 436. This most wonderful object has the appearance of fig. 4, 

 plate 5. The southern head is denser than the northern. The light of this 

 object is perfectly milky. There are one or two stars in it. 



NOTE 229, p. 436. Fig. 5, plate 5, represents this brother system. 



NOTE 230, p. 436. Fig. 6, plate 5, represents one of the spindle-shaped 

 nebulae. 



NOTE 231 , p. 449. Elongation. The apparent angular distance of an object 

 from the centre of the sun. 



