Theory of Colours
In the 18th century, Johann Wolfgang von Goethe wrote about optical spectra in his "Theory of Colours". Goethe used the word "spectrum" ("Spektrum") to designate a ghostly optical afterimage, as did Schopenhauer in "On Vision and Colors". Goethe argued that the continuous spectrum was a compound phenomenon. Where Newton narrowed the beam of light to isolate the phenomenon, Goethe observed that a wider aperture produces not a spectrum but rather reddish-yellow and blue-cyan edges with white between them. The spectrum appears only when these edges are close enough to overlap.