NOTES. 433 



tureen the circle and the parabola, and an infinity of hyperbolas II P Y 

 exterior to the parabola p P R. 



NOTE 223, p. 371 .Let A B, fig. 26, be the diameter of the earth's orbit, 

 and suppose a star to be seen in the direction A S' from the earth when 

 at A. Six months afterward, the earth having moved through half of 

 its orbit, would arrive at B, and then the star would appear in the direc- 

 tion B S', if the diameter A B, as seen from S', had any sensible magni- 

 tude. But A B, 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, sup- 

 posed to be the nearest of the fixed stars. 



NOTE 224, p. 373. 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 CD O, fig. 14, for 

 the sake of illustration, the earth being in d. It is clear that, when the 

 star moves through C D O, its light will take longer in coming to the earth 

 from O than from C, by the whole time it employs in passing^hrough 

 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 prod A will be the breadth of 

 the orbit hi miles. From this the dimensions of the ellipse will be ob- 

 tained by the aid of observation, the length and position of any diameter, 

 as Sp, may be found ; and as nil the angles of the triangle d Sp can be 

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

 be computed. 



NOTE 225, p. 376. One of the globular clusters mentioned in the text 

 is represented in fig. 1, plate 5. The stars are gradually condensed to- 

 ward the center, where they run together into a blaze somewhat like a 

 snowball. The more condensed part is projected on a ground of irregu- 

 larly-scattered stars, which fills the whole field of the telescope. There 

 are few stars in the neighborhood of this cluster. 



NOTE 226, p. 378. Fig. 2, plate 5, represents one of those enormous 

 rings in its oblique position. It has a dark space in the center, with a 

 small star at each extremity. 



NOTE 227, p. 378. Fig. 3, plate 5, may convey some idea of the ring 

 in the constellation of the Lyre mentioned hi the text. 



NOTE 228, p. 378. 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. 378. Fig. 5, plate 5, represents this brother system. 



NOTE 230, p. 379. Fig. 6, plate 5, represents one of the spindle-shaped 

 nebulae. 



NOTE 231, p. 385. Elongation. The apparent angular distance of an 

 object from the center of the sun. 



28 Oo 



