  An extrasolar planet can be detected via microlensing from the perturbation
it makes in the smooth lensing light curve of the primary. In addition to the
conventional photometric microlensing, astrometric observation of the
center-of-light motion of the source star image provides a new channel of
detecting and characterizing extrasolar planets. It was known that the
planet-induced astrometric signals tend to be positive while the photometric
signals can be either positive or negative. In this paper, we analytically show
the reason for these tendencies of microlensing planetary signals.
