Quasar Image Shifts Resulting from Gravitational Microlensing

Gravitational microlensing of quasars by stars in external galaxies can introduce uctuations in the centroid of the "" pointlike macroimages. The induced shifts are extremely small, on microarcsecond scales, below the limits of current optical observations. However, such shifts will become measurable with the proposed Space Interferometry Mission, due to y in 2005. The degree of the centroid shifts and their application as probes of both quasar structure and the stellar mass function in the lensing galaxy are discussed.