The science of measuring the polarization of light from celestial objects at narrower bandwidths than is possible with a conventional polarimeter. The principles are the same as with a conventional polarimeter, but the wavelengths measured are isolated with a spectrograph rather than with filters. Spectropolarimetry is used to measure the magnetic fields of stars from the Zeeman effect and to study the origin of spectral lines. Some instruments can measure all four Stokes parameters from individual atomic lines, allowing measurement of longitudinal and transverse fields and the direction of the polarization vector on the sky. Other instruments are designed to measure the Zeeman effect in the wings of hydrogen Balmer lines, allowing detection of fields in much fainter objects.