The article deals with the discovery of the invisible rays of the solar spectrum. In the years 1775-1777, some natural philosophers carried out investigations on the thermal and chemical properties of different kinds of rays. Independently, Landriani and Rochon decomposed a ray of sunlight by a prism and found that different rises in temperature were produced at different places of the spectrum. In 1777, Scheele published the results of his research on the chemical reactions of light on silver chloride. All these scientists failed to extend their investigations to the invisible regions beyond the red and the violet ends of the spectrum. The invisible heat rays in the solar spectrum were discovered by Herschel in 1800. In 1801, Ritter found the ultraviolet rays. Strongly influenced by the philosophical views of Romanticism and Naturphilosophie, he believed in the principle of polarity in nature, and after Herschel’s discovery, he hypothesized a possible polarity in the spectrum and successfully looked for invisible radiation beyond the violet. The reception of this discovery was hindered by Ritter’s abstruse style and his tendency to mix speculations with scientific observations.