The Long-Wavelength Spectrometer (LWS) is one of two complementary spe
ctrometers aboard the European Space Agency's Infrared Space Observato
ry(1) (ISO) (Kessler et al., 1996). It operates over the wavelength ra
nge 43 - 196.9 mu m at either medium (about 150 to 200) or high (6800
to 9700) spectral resolving power. This Letter describes the instrumen
t and its modes of operation. a companion paper (Swinyard et al, 1996)
describes its performance and calibration.