Computed by J. Fletcher, Esq., 1853.
Note 233, [p. 403]. The mass is found in the manner explained in the text; 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 C D 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 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 product 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 S p may be found; and as all the angles of the triangle d S p can be determined by observation, the distance of the star from the earth may be computed.
Note 234, [p. 405]. The mean results of MM. Argelander, Otto Struve, and Luhndahl for stars in the northern hemisphere and the epoch 1790, places the point to which the sun is tending in 259° 5ʹ of right ascension and 55° 23ʹ of north polar distance. Mr. Gallaway computed from stars in the southern hemisphere, at the same epoch, the point to have been in 260° 1ʹ right ascension and 55° 37ʹ north polar distance, results nearly identical, though from very different data.
Note 235, [p. 414]. One of the globular clusters mentioned in the text is represented in fig. 1, [plate 8]. The stars are gradually condensed towards the centre, where they run together in a blaze. 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 near this cluster.
Note 236, [p. 420]. [Plate 8] shows five nebulæ as seen in Sir John Herschel’s 20-feet telescope.
1. An enormous ring seen obliquely with a dark centre and a small star at each extremity.
2. The ring in the constellation Lyra.
3. The dumb-bell nebula in Vulpicula.
4. The spiral nebula or brother system in the 20-feet telescope.
5. A spindle-shaped nebula.