OF THE COSMOS. PERIODICALLY VARIABLE STARS. 163 



naked eye, or, in stars not visible to the naked eye, with 

 a Fraunhofer's Comet-seeker of 24 Parisian inches focal 

 length. For the brighter stars above the 6th magnitude, 

 a gradation is about the 10th part of the difference 

 between two successive orders of magnitude ; for the 

 smaller stars, the magnitudes in ordinary use are con- 

 siderably closer together. 



1. o Ceti, E. A. 32 57', Decl.-3 40'; also called 

 Mira, on account of its wonderful change of light, the 

 phenomenon having been first observed in this star. The 

 periodicity of the change was already recognised in the 

 second half of the 17th century, and Bouillaud determined 

 the duration of the period at 333 days ; it was also found 

 at the same time that this duration was sometimes longer 

 and sometimes shorter, as well as that the light of the 

 star, when at the greatest, was sometimes brighter and 

 sometimes fainter. This has since been perfectly confirmed. 

 Whether the star ever becomes quite invisible has not yet 

 been decided; it has sometimes been seen of the llth or 

 12th magnitude at the time of the minimum, and at other 

 times it has not been possible to see it with 3 and 4 feet 

 telescopes. Thus much is certain, that it is for a long 

 time fainter than the 10th magnitude. There are, however, 

 few existing observations of it at this stage ; most obser- 

 vations commencing only when, being of the 6th mag- 

 nitude, it begins to shew itself to the naked eye. From 

 that moment the star increases in brightness, at first ra- 

 pidly, then more slowly, and afterwards more rapidly. 

 On the mean, the time occupied by the increase of light, 

 from the 6th magnitude upwards, is 50 days, and by the 

 decrease of light, down to the same degree of brightness, 



