A far ir interferometric spectrometer with a special electronic computer has been constructed. The interferometer is of the conventional Michelson type. The electronic computer, which consists of an analog computer to introduce apodization and a digital computer to calculate the Fourier cosine transform, is fed with the detector output of the interferometer and is operated by electric pulses from a moiré fringe counter that is part of the mechanism that measures the path difference in the interferometer. The digital computer makes the calculation in parallel for each of one thousand spectral positions during the scanning of the interferogram. It takes 0.45 sec for the calculation at each path difference. A spectrogram can be obtained in 10 min; this includes calculations at one thousand path differences and 2 min for recording. During the scanning, values stored in the memory corresponding to the one thousand spectral positions can be displayed on a synchroscope in order to provide a spectrogram that is low in resolving power at the beginning and high at the end of the scan. Some results obtained with water vapor absorption are shown.