Variation of Lacaille 5861 43 



variation. To any such inquiry unfortunately no definite answer 

 can be given. 



We can only assure ourselves of a series of negations. 



1. The light variation of L 5861 is not due to eclipse. 



If that were so there would be periods of rest, during which the 

 light of the star would remain constant. But no such stationary 

 periods have been observed. 



2. L 5861 cannot be a large star. 



If so the two possible theories of light variation, rotation and 

 revolution, would not be tenable. The latter could not exist ; two 

 large bodies could not revolve round one another in six hours ; the 

 former would end in the disruption of the star. 



3. The light variation of L 5861 arises from some mechanical, not 

 chemical source. 



The period of maxima and minima are of extreme regularity, and 

 the limits or amplitude of variation is of a certain fixed lati- 

 tude. With variables, where spectroscopic examination indicates 

 chemical change synchronous with the light variations, regularity 

 of period and of amplitude is wanting, as we might naturally 

 expect. 



No chemical changes through unstable combination of the con- 

 stituents of the star's upper atmosphere could alternate with the 

 precision of an observatory clock. 



4. The changes are not deep-seated. 



No body could, as a whole, alternate through extremes of light 

 and shade in periods of six hours. The changes are consequently 

 surface phenomena, probably tidal in their nature. An upper 

 viscous shell covering a solid globe would, under tidal forces, 

 generate by friction enough heat at high tide to cause the change of 

 light observed on either L 5861 or U Pegasi. 



The conditions of variation would be, in that case, two stars 

 revolving round one other in close proximity in a period of twelve 

 or fourteen hours. The mutual attractions of the two spheres 

 would produce two giant tides in each star, which as they passed 

 over the more solid nucleus underneath would generate heat, and 

 so give rise to an increase of brightness. 



An examination of the star's relative position might indicate a 

 sensible parallax, and thus we might be able to compare its light 

 with that of stars whose distance is known. 



The whole subject of stellar variation is one of no common 

 interest ; and now that spectroscopic researches at Potsdam and 

 Harvard have established a relationship between stellar variation 

 and double stars, the interest is accentuated. 



