A spectroheliograph is an instrument used to photograph the Sun at the wavelength of a strong Fraunhofer line, usually the hydrogen-alpha or calcium K line. The image of the Sun formed by a telescope is focused onto the slit of a high-dispersion spectrograph. A second slit, immediately below the first and lying in the spectrograph's focal plane, isolates the chosen narrow band of wavelengths from the solar continuum. By moving the primary slit across the Sun's image and moving the secondary slit to keep its spectral location, a monochromatic photograph of the Sun is obtained. A spectrohelioscope works in the same way, but visually rather than photographically. By placing the eye instead of a photographic plate behind the second slit, and then vibrating both slits in unison through a small amplitude, a monochromatic image of part of the solar disk can be seen. The image appears steady because of the persistence of vision.