164 SPECIAL RESULTS IN THE URANOLOGICAL PORTION 



69 days; so that the star is visible to the naked eye for 

 an interval of about four months. This is, however, only the 

 mean duration of the star's visibility, which has sometimes 

 been augmented to five, and sometimes diminished to only 

 three months. So, also, the relative duration of the increase 

 and decrease of light is subject to great fluctuations, the 

 former being sometimes slower than the latter : as was 

 the case in 1840, when the star took 62 days to arrive at 

 its greatest brightness, and in 49 days decreased from 

 thence to invisibility to the naked eye. The shortest ob- 

 served duration of the increase was SO days, in 1679 ; the 

 longest, 67 days, in 1709. The decrease lasted longest 

 in 1839, when it extended over 91 days, and was shortest 

 in 1660, when it was only 52 days. Sometimes, at the 

 time of its greatest brightness, the light of the star 

 scarcely undergoes any sensible change in the course of 

 an entire month ; at other times an alteration is distinctly 

 perceptible at the end of a few days. Sometimes, after 

 the star has decreased in brightness for some weeks, a 

 suspension of change for several days ensues, or at least 

 the decrease becomes scarcely sensible : this was the case 

 in the years 1678 and 1847. 



As already noticed, the maximum brightness is by no 

 means always the same. If we represent the light of th 

 faintest star visible to the naked eye by 0, and that of 

 Aldebaran, a star of the first magnitude, by 50, then the 

 observed maximum brightness of Mira has fluctuated 

 between 20 and 47, i. e. between the brightness of stars 

 of the 4th and of the 1st to the 2nd magnitudes : the 

 mean brightness is 28, or that of the star y Ceti. The 

 duration of the period has been almost even more irregular : 



