A METHOD of making thin selenium films with free surfaces has been devised, and this has enabled us to construct a pile of selenium films which polarize a beam of radiation falling on the plates at the appropriate angle. The transmitted beam is the one which is used, and a high degree of polarization (with very little loss of efficiency) is obtained with only three films. The films are not perfectly flat ; since, however, they are only a few microns thick, this introduces no distortion in the transmitted beam, and is of service in eliminating the loss of polarization due to multiple reflexions. The films show a small absorption at a wave-length of 2 µ, but are otherwise transparent so far as we have examined them (about 7 µ).