Lensing of Stars by Spherical Gas Clouds
  If the Galaxy contains ~10^{11}M_sol in cold gas clouds of ~Jovian mass and \~AU size, these clouds will act as converging lenses for optical light, magnifying background stars at a detectable rate. The resulting light curves can resemble those due to gravitational lensing by a point mass, raising the possibility that some of the events attributed to gravitational microlensing might in fact be due to ``gaseous lensing''. During a lensing event, the lens would impose narrow infrared and far-red H_2 absorption lines on the stellar spectrum. Existing programs to observe gravitational microlensing, supplemented by spectroscopy, can therefore be used to either detect such events or place limits on the number of such gas clouds present in the Galaxy. 
B. T. Draine (Princeton University Observatory)