CHAPTERS ON THE STARS. 641 



easily be noticed by any one who looks for them at the right times, and 

 knows how to find the stars. These stars are: 



Omicron Ceti, called also Mira Ceti. 

 Beta Persei, or Algol. 

 Beta Lyrae. 



It happens that each of these stars exemplifies a certain type or law 

 of variations. 



Omicron Ceti. On August 13, 1596, David Fabricius noticed a star in 

 the constellation Cetus, which was not found in any catalogue. Bayer, 

 in his 'Uranometria', of which the first edition was published in 

 1601, marked the star Omicron, but said nothing about the fact that 

 it was visible only at certain times. Fabricius observed the star from 

 time to time, until 1609, but he does not appear to have fully and 

 accurately recognized its periodicity. But so extraordinary an object 

 could not fail to command the attention of astronomers, and the fact 

 was soon established that the star appeared at intervals of about eleven 

 months, gradually fading out of sight after a few weeks of visibility. 

 Observations of more or less accuracy having been made for more than 

 two centuries, the following facts respecting it have been brought to 

 light: 



Its variations are somewhat irregular. Sometimes, when at its 

 brightest, it rises nearly or quite to the second magnitude. This was 

 the case in October, 1898, when it was about as bright as Alpha Ceti. At 

 other times its maximum brightness scarcely exceeds the fifth magni- 

 tude. No law has yet been discovered by which it can be predicted 

 whether it shall attain one degree of brightness or another at maximum. 



Its minima are also variable. Sometimes it sinks only to the eighth 

 magnitude; at other times to the ninth or lower. In either case it is 

 invisible to the naked eye. 



As with other stars of this kind, it brightens up more rapidly than 

 it fades away. It takes a few weeks from the time it becomes visible 

 to reach its greatest brightness, whatever that may be. It generally 

 retains this brightness for two or three weeks, then fades away, gradu- 

 ally at first, afterward more rapidly. The whole time of visibility will, 

 therefore, be two or three months. Of course, it can be seen with a 

 telescope at any time. 



The period also is variable in a somewhat irregular way. If we 

 calculate when the star ought to be at its greatest brightness on the 

 supposition that the intervals between the maxima ought to be equal, 

 we shall find that sometimes the maximum will be thirty or forty days 

 early, and at other times thirty or forty days late. These early or late 

 maxima follow each other year after year, with a certain amount of 

 regularity as regards the progression, though no definable law can be 



VOL. LVII.-4] 



