Three-dimensional Microlensing
  We study three-dimensional microlensing where two lenses are located at
different distances along the line of sight. We formulate the lens equation in
complex notations and recover several previous results. There are in total
either 4 or 6 images, with an equal number of images with positive and negative
parities. We find that the sum of signed magnifications for six image
configurations is unity. Furthermore, we show that the light curves can be
qualitatively different from those for binary lensing in a single plane. In
particular, the magnifications between a `U'-shaped caustic crossing can be
close to unity, rather than having a minimum magnification of 3 as in the
single plane binary lensing. There is only a small probability
three-dimensional microlensing events will be seen in microlensing toward the
Galactic centre. It is more likely they will be first seen in cosmological
microlensing.
