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or less complex systems of bodies having the widest diversity in their construction. Double stars have been familiar to every observer of the heavens since the time of the great Herschel. But it is only in the time of our generation that the spectroscope has begun to make known to us pairs of stars revolving round each other, of which the components are so close together that the most powerful telescope can never separate them. The history of science offers no greater marvel than the discoveries of invisible planets moving round many of the stars which are now being made, and in which the Lick observatory has recently taken the lead.

It now seems more or less probable that the changes of light in all stars having a regular and constant period is due to the revolution of large planets or other stars around them. Sometimes the variation is slight and is caused in the way we have described, by one body partially eclipsing the other as it passes across it. In this case there may be no real variation in the light; the star eclipsed shines just as bright behind the eclipsing body as when it is not eclipsed. But it now seems that, if the darker body revolves in a very eccentric orbit, so as to be much nearer the bright body at some times than at others, its attraction produces such a change in the other as to greatly increase its light. Just how this effect is produced it is as yet impossible to say.