An optical instrument for forming the spectrum of a light source and recording it on a film. The dispersing medium may be a prism or a diffraction grating. A concave grating requires no other means to form a sharp image of the slit on the film, but a plane grating or a prism requires auxiliary lenses or concave mirrors to act as image-forming means in addition to the dispersing element. Refracting prisms can be used only in parallel light, so a collimating lens is required before the prism and an imaging lens after it. Some special-purpose spectrographs are equipped with tiny photodetectors situated in the spectrum at the positions corresponding to the lines of elements whose presence is to be determined.