A plane rectangular mirror is placed in front of the grating of a spectrograph. This mirror returns the parallel radiation n (e.g. 10) times back to the grating. In this way, the resolution can improve by the same factor. A resolving power of 10⁶ has been demonstrated by a 105 mm grating with 1200 grooves/mm, using n=4. Using 10 passes instead, such a resolving power would also be feasible with a smaller grating and a 70 cm spectrograph.