Transmissions and reflections have been measured at nine wave‐lengths between 20.7&mgr; and 152&mgr; for a number of plastics and special surfaces. The residual rays from crystals were used as sources of approximately monochromatic radiation. The general purpose of the investigation is to find materials for absorbing and transmitting filters to be used in the far infra‐red region of the spectrum. The spectroscopy of the far infra‐red depends primarily upon the use of absorbing filters and reflecting surfaces to obtain purity of spectra. Relatively few materials are suitable for these purposes and additional materials are greatly needed. Vacuum spectrographs and absorption cells require transparent windows capable of standing pressure. It is difficult to avoid using less than four such windows in any spectroscopic arrangement. If the transmission of the individual windows can be raised from 60 percent to 80 percent the transmission of the four is increased by a factor of more than three. Similarly it is a matter of importance that the absorbing material on the receiving surfaces of the thermopile be efficient in transforming incident radiant energy into heat. The satisfactory solution of these technical requirements in the far infra‐red is a much more difficult one than in the near infra‐red, with the result that extended investigations to discover materials suitable for these various purposes cannot well be avoided. The results recorded here deal primarily with plastics though a number of other substances are included.