In the 19th century new developments such as the discovery of photography, Rowland's invention of the concave diffraction grating, and Schumann's works on discovery of vacuum ultraviolet (fluorite for prisms and lenses, low-gelatin photographic plates and absorption of UV in air below 185 nm) made advance to shorter wavelengths very fast. At the same time Dewar observed series in alkali spectra, Hartley found constant wave-number differences, Balmer discovered a relation connecting wavelengths in the visible hydrogen spectrum, and finally Rydberg derived a formula for wave-numbers of spectral series. Meanwhile, the substantial summary of past experiments performed by Maxwell (1873), resulted in his equations of electromagnetic waves.