We may remark, in conclusion, that though there are quite a number of variable stars, their nature and the cause of their changes are but imperfectly understood.  The Star of Bethlehem has no doubt an orbit, which brings it much nearer the earth at some times than others.  But astronomers do not believe that the mere fact of distance explains all changes.  There is a star known as Mira, which for eleven months is wholly invisible to the naked eye, then flames forth as a star of the first magnitude, and is visible for a period of nearly three months, fading at its close into darkness again.  The star Algol, in the constellation Perseus, is usually of the second magnitude, but every two and a-half days it begins to decline in brilliancy, becomes very faint, and remains thus for about three hours, and then waxes bright again.  Possibly this may be caused by the shadow of another star.  In 1866 a star of the eighth magnitude, in the Northern Crown, suddenly flamed up into extraordinary brilliancy, remained thus for several months and gradually subsided.  This star was examined with the spectrum, and showed lines of burning hydrogen.  This led to the theory, now held, that the increase in brilliancy of these stars is caused by the incandescence of this gas.  These fixed stars are all supposed to be suns of other systems, and to be surrounded—­like our sun—­with envelopes of fiery gases; from some cause not at all understood these gases may, at regular periods, flame up with fiercer heat than usual, and produce this appearance of greatly increased light.  This is a very inadequate explanation, no doubt, but it is the best that astronomers have yet been able to devise in the matter.

    A.C.C.

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