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the fact that the star is a double one, composed of two stars revolving around each other, and so near together as almost to touch. As they revolve, each one in succession wholly or partially hides the other. This fact is not brought out by the telescope, because the most powerful telescope that could be made would not show the two stars separately. It is the result of long and careful study of the spectrum of the star, which is found to be a double one, the lines in one of which alternately cover and recede from the lines of the other.

In the extent of variation of its light the most remarkable of the more conspicuous variable stars is Omicron Ceti, already mentioned as seen by Fabritius. It is now found to go through a regular period in three hundred and thirty days. During about two weeks of this time it is at its brightest, and is then sometimes of the second magnitude and sometimes much fainter—occasionally only of the fifth. After each maximum it gradually fades away for a few weeks and disappears from view to the naked eye. But with a telescope it can be seen all the year round.

The period of eleven months makes the maximum occur about a month earlier every year. During some years it will occur when the star is so near the sun that it cannot be easily observed. This will be the case during the years 1903-'05.

Algol, also called Beta Persei, being in northern declination, can be seen in our latitudes at some time on almost every night of the year. In autumn and winter it is visible in the early evening. The peculiarity of its