234 COSMOS. 



dnys; it was found, however, at the same time, that this dura- 

 tion was sometimes longer, and sometimes shorter, and that 

 the star at its greatest brilliancy appeared sometimes brighter, 

 and sometimes fainter. This has been subsequently fully 

 confirmed. Whether the star ever becomes perfectly invisible 

 is as yet undecided ; at one time, at the epoch of its minimum 

 it has been observed of the 1 1th or 12th magnitude, at another, 

 it could not be seen even with the aid of a three or a four- 

 feet telescope. This much is certain, that for a long period 

 it is fainter than stars of the 10th magnitude. But few ob- 

 servations of the star at this stage have as yet been taken ; 

 most having commenced when it had begun to be visible 

 to the naked eye as a star of the 6th magnitude. From this 

 period the star increases in brightness at first with great 

 rapidity, afterwards more slowly, and at last, with a scarcely 

 perceptible augmentation ; then again, it diminishes at first 

 slowly, afterwards rapidly. On a mean the period of aug- 

 mentation of light from the 6th magnitude extends to 50 days; 

 that of its decrease down to the same degree of brightness 

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

 about four months. However, this is only the mean duration 

 of its visibility; occasionally it has lasted as long as five 

 months, whereas, at other times it has not been visible for 

 more than three. In the same way, also, the duration both 

 of the augmentation and of the diminution of its light is 

 subject to great fluctuations, and the former is at all times 

 slower than the latter: as, for instance, in the year 1840, 

 when the star took sixty-two days to arrive at its greatest 

 brightness, and then in forty-nine days became invisible to 

 the naked eye. The shortest period of increase that has as 

 yet been observed took place in 1679, and lasted only thirty 

 days; the longest (of sixty-seven days) occurred in 1709. 

 The decrease of light lasted the longest in 1839, being then 

 ninety-one days; the shortest in the year 1660, when it was 

 completed in nearly fifty-two days. Occasionally, the star 

 at the period of its greatest brightness exhibits for a whole 

 month together scarcely any perceptible variation ; at others. 

 a difference may be observed within a very few days. On 

 some occasions after the star had decreased in brightness for 

 several weeks there was a period of perfect cessation ; or, at 

 least, a scarcely perceptible diminution of light during several 

 days: this was the case in 1678 and in 1847. 



