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Rh by mathematicians. It is twice as heavy as our sun, but does not give a fiftieth part of the light. Stars then may be double, or treble, or even quadruple by nature, or by the accident of position.

Comparing his own observations and such others as he could procure, Herschel calculated that the one star moved round the other, or that both moved round their common centre of gravity in the following double stars:—

Another double star that he carefully examined was Zeta Herculis. It presented him with a sight "which is new in astronomy; it is, the occultation of one star by another." For twenty-one years he continued to keep a watch on the star. After twenty years had passed he could no longer perceive the smaller of the two companions. The following year he found "the apparent disk a little distorted; but there could not be more than about $3⁄8$ of the apparent diameter of the small star wanting to a complete occultation." But the observations made were not sufficient to determine the nature of the motion that produced these effects. The long period of 1681 years set down against ε Bootis, Herschel himself points out as subject to uncertainties, which it will take long to clear up.