GEOMETRIC DISTANCE MEASUREMENT .2.

Sparks proposed a geometric way to measure the distances of galaxies, from imaging polarization observations of supernova Light echoes. Here , a simple extension of that method of geometric distance determinatio n is described: measurement of the proper motion of polarized light ec hoes. This requires measurement at more than one epoch, unlike the ori ginal technique, and is technically more difficult. However, in princi ple, it is far more widely applicable. Measurement of high polarizatio n from scattered light (i.e., a light echo) constrains the scattering angle to be close to 90 degrees and so the intrinsic speed of motion o f the echo will be close to c transverse to the line of sight. If the proper motion, i.e., apparent angular speed, is obtained, then the dis tance follows immediately. Scattered light from any transient or highl y variable source of illumination, such as active galactic nuclei or n ovae as well as supernovae, may be used in this case.