The Michelson interferometer flown on Nimbus III in April 1969 has obtained infrared emission spectra of the earth and its atmosphere within 400 cm−1 and 2000 cm−1 (5 μ and 25 μ). Spectra of good quality have been recorded with a spectral resolution corresponding to 5 cm−1. This paper discusses the design of the instrument including the optical layout, the phase locked loop operation of the Michelson motor, and the functioning of the reference interferometer. The methods of data reduction and in-flight calibration are demonstrated on sample spectra recorded while in orbit around the earth.