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light. As observers studied the heavens with greater care, more and more of such stars were found, until at the present time the list of them numbers four or five hundred, and is constantly increasing. Of these some vary in an irregular way, but a large majority go through a regular period.

The easiest of these objects to notice is Beta Lyra, which is marked B on the figure of that constellation already given. It can be seen at some hour of any clear evening, spring, summer, or autumn. If the reader as he takes his evening walk will, night after night, compare this star with the one nearest to it and nearly of the same magnitude, he will see that while on some evenings the two appear perfectly equal, on others Beta will be of a magnitude fainter than the other. Careful and continued watching will show that the change takes place in a period of about six days and a half. That is to say, if the two stars are equal on a certain evening, they will again appear equal at the end of six or seven days, and so on indefinitely. Midway between the two times of equality the variable one will be at its faintest. If the observer notes the magnitudes at this time with the greatest precision, a curious fact will be brought out. Every alternate minimum, as the phase of least light is called, is slightly fainter than that preceding or following. The actual period is therefore nearly thirteen days, during which time there are two maxima of equal brightness and two slightly different minima.

It is now known that the variation of light in this case is not really inherent in the star itself, but arises from