ASTRONOMY WITH AN OPERA-GLASS. 67 



is to assume that it is a variable star of long period, and possessing a 

 very wide range of variability. One significant fact that would seem 

 to point to some connection between star and the nebula, after all, is 

 that a similar occurrence was noticed in the constellation Scorpio in 

 1860, and to which I have previously referred (see " Popular Science 

 Monthly-' for June, 1887). In that case a faint star projected against 

 the background of a nebula, suddenly flamed into comparatively 

 great brilliancy, and then faded again. The chances against the 

 accidental superposition of a variable star of such exti-eme variability 

 upon a known nebula occurring twice are so great that for that reason 

 alone we might be justified in thinking some mysterious causal rela- 

 tion must in each case exist between the nebula and the star. The 

 temptation to indulge in speculation is very great here, but it is better 

 to wait for more light, and confess that for the present these things 

 are inexplicable. 



It will be found very interesting to sweep with the glass slowly 

 from side to side over Andromeda, gradually approaching toward Cas- 

 siopeia or Perseus. The increase in the richness of the stratum of 

 faint stars that apparently forms the background of the sky will be 

 clearly discernible as you approach the Milky- Way, which passes di- 

 rectly through Cassiopeia and Perseus. It may be remarked that the 

 Milky-Way itself, in that splendidly rich region about Sagittarius 

 (described in the "Stars of Summer"), is not nearly so effective an 

 object with an opera-glass as it is above Cygnus and in the region 

 with which we are now dealing. This seems to be owing to the 

 smaller magnitude of its component stars in the southern part of the 

 stream. There the background appears more truly " milky," while in 

 the northern region the little stai's appear distinct, like diamond- specks 

 on a black background. 



The star Nu, which serves as a pointer to the Great Nebula, is 

 itself worth some attention with a pretty strong glass on account of a 

 pair of small stars near it. 



Next let us turn to Perseus. The bending row of stars marking 

 the center of this constellation is very striking and brilliant. The 

 brightest star in the constellation is Alpha, or Algenib, in the center 

 of the row. The head of Perseus is toward Cassiopeia, and in his left 

 hand he grasps the head of Medusa, which hangs down in such a way 

 that its principal star Beta, or Algol, forms a right angle with Algenib 

 and Almaach in Andromeda. This star Algol, or the Demon, as the 

 Arabs call it, is in some respects the most wonderful and interesting 

 in all the heavens. It is as famous for the variability of its light as 

 Mira, but it differs widely from that star both in its period, Avhich is 

 very short, and in the extent of the changes it undergoes. During 

 about two days and a half, Algol is equal in brilliancy to Algenib, 

 which is a second-magnitude star ; then it begins to fade, and in the 

 course of about four and a half hours it sinks to the fourth magnitude, 



