RITTER, (JOHANN W.). - THE DISCOVERY OF ULTRAVIOLET LIGHT.

Versuche über das Sonnenlicht.

Halle, Rengerschen Buchhandlung, 1803. Without wrappers as extracted from "Annalen der Physik. Herausgegeben von Ludwig Wilhelm Gilbert", Jahrgang 1802, Bd. 12, Zwölftes Stück. Pp. 409-416. Titlepage to vol. 12.

This is Ritter's first expositon of his discovery of ultraviolet light. It was announced the year before in a halfpage letter addressed to Gilbert's Annalen and printed in the Annalen. With that discovery, it became clear that visible light represents no more than a fraction of a continous spectrum.


A year earlier, in 1800, William Herschel discovered infrared light. This was the first time that a form of light beyond visible light had been detected. After hearing about Herschel's discovery of an invisible form of light beyond the red portion of the spectrum, Ritter decided to conduct experiments to determine if invisible light existed beyond the violet end of the spectrum as well. He had heard that blue light caused a greater reaction in silver chloride than red light did. Ritter decided to measure the rate at which silver chloride reacted to the different colors of light. He directed sunlight through a glass prism to create a spectrum. He then placed silver chloride in each color of the spectrum and found that it showed little change in the red part of the spectrum, but darkened toward the violet end of the spectrum. Johann Ritter then decided to place silver chloride in the area just beyond the violet end of the spectrum, in a region where no sunlight was visible. To his amazement, this region showed the most intense reaction of all. This showed for the first time that an invisible form of light existed beyond the violet end of the visible spectrum. This new type of light, which Ritter called Chemical Rays, later became known as ultraviolet light or ultraviolet radiation (the word ultra means beyond).

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