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might be due to an eclipse of the brilliant star by some dark companion revolving about it; indeed, this theory seemed to hold the field, inasmuch as its only rival was one which supposed Algol to be a rotating body darker on one side than the other.

Fig. 29.—Orbit of the Companion of Algol.

This, however, was easily shown to be incompatible with the observed facts as to the manner in which the light waxed and waned in a single cycle of change. It was, however, impossible to subject the eclipse theory to any decisive test until astronomers were provided with the means of measuring the velocity of approach or retreat along the line of sight. The existence of the dark companion was therefore almost