"In the paper in 1823, Fraunhofer revealed his continued invstigation of diffraction gratings. Using a diamond point, he could rule up to 3,200 lines per Paris inch. He continued his study of the effect of oblique rays, developed formulations based on the wave conception, and calculated a revised set of wavelenghts for the major spectral lines. Thus, his earlier observations of the dark lines in the solar spectrum enabled him to make the highly precise measurements of dispersions; then his use of the wave theory of light allowed him to derive, with suitable simplifications, the general formulation of the grating equation still in use today." (Reese V. Jenkins in DSB).