PLEASURES OF THE TELESCOPE. 519 



magnitude, but at maximum it seldom exhibits the greatest bril- 

 liance that it has on a few occasions shown itself capable of attain- 

 ing. Ordinarily it begins to fade after reaching the fourth or 

 fifth magnitude. The period averages about three hundred and 

 thirty-one days, but is irregularly variable to the extent of 

 twenty-five days. Its color is red, and its spectrum shows bright 

 lines, which it is believed disappear when the star sinks to a mini- 

 mum. Among the various theories proposed to account for such 

 changes as these the most probable appears to be that which 

 ascribes them to some cause analogous to that operating in the 

 production of sun spots. The outburst of light, however, as 

 pointed out by Scheiner, should be regarded as corresponding to 

 the maximum and not the minimum stage of sun spot activity. 

 According to this view, the star is to be regarded as possessing an 

 extensive atmosphere of hydrogen, which, during the maximum, 

 is upheaved into enormous prominences, and the brilliance of the 

 light from these prominences suffices to swamp the photospheric 

 light, so that in the spectrum the hydrogen lines appear bright 

 instead of dark. 



It is not possible to suppose that Mira can be the center of a 

 system of habitable planets, no matter what we may think of the 

 more constant stars in that regard, because its radiation manifestly 

 increases more than six hundred fold, and then falls off again to 

 an equal extent once in every ten or eleven months. I have met 

 people who can not believe that the Almighty would make a sun 

 and then allow its energies " to go to waste," by not supplying it 

 with a family of worlds. But I imagine that if they had to live 

 within the precincts of Mira Ceti they would cry out for exemp- 

 tion from their own law of stellar usefulness. 



The most beautiful double star in Cetus is y, magnitudes third 

 and seventh, distance 3", p. 288 ; hues, straw-color and blue. The 

 leading star a, of magnitude two and a half, has a distant blue 

 companion three magnitudes fainter, and between them are two 

 minute stars, the southernmost of which is a double, magnitudes 

 both eleven, distance 10", p. 225. 



The variable S ranges between magnitudes seven and twelve 

 in a somewhat irregular period of about eleven months, while R 

 ranges between the seventh and the thirteenth magnitudes in a 

 period of one hundred and sixty-seven days. 



The constellation Eridanus, represented in map No. 21, con- 

 tains a few fine double stars, one of the most interesting of which 

 is 12, a rather close binary. The magnitudes are fourth and 

 eighth, distance 2", p. 327. We shall take the five-inch for this, 

 and a steady atmosphere and sharp seeing will be necessary on 

 account of the wide difference in the brightness of the component 

 stars. Amateurs frequently fail to make due allowance for the 



